feat: email notifications + deadline reminder system

Database:
- notification_preferences table (user_id, tenant_id, reminder days, email/digest toggles)
- notifications table (type, entity link, read/sent tracking, dedup index)

Backend:
- NotificationService with background goroutine checking reminders hourly
- CheckDeadlineReminders: finds deadlines due in N days per user prefs, creates notifications
- Overdue deadline detection and notification
- Daily digest at 8am: compiles pending notifications into one email
- SendEmail via `m mail send` CLI command
- Deduplication: same notification type + entity + day = skip
- API: GET/PATCH notifications, unread count, mark read/all-read
- API: GET/PUT notification-preferences with upsert

Frontend:
- NotificationBell in header with unread count badge (polls every 30s)
- Dropdown panel with notification list, type-colored dots, time-ago, entity links
- Mark individual/all as read
- NotificationSettings in Einstellungen page: reminder day toggles, email toggle, digest toggle

frontend/src/app/(app)/einstellungen/page.tsx

@@ -1,11 +1,12 @@
 "use client";
 
 import { useQuery } from "@tanstack/react-query";
-import { Settings, Calendar, Users } from "lucide-react";
+import { Settings, Calendar, Users, Bell } from "lucide-react";
 import Link from "next/link";
 import { api } from "@/lib/api";
 import type { Tenant } from "@/lib/types";
 import { CalDAVSettings } from "@/components/settings/CalDAVSettings";
+import { NotificationSettings } from "@/components/settings/NotificationSettings";
 import { SkeletonCard } from "@/components/ui/Skeleton";
 import { EmptyState } from "@/components/ui/EmptyState";
 
@@ -97,6 +98,19 @@ export default function EinstellungenPage() {
             </div>
           </section>
 
+          {/* Notification Settings */}
+          <section className="rounded-xl border border-neutral-200 bg-white p-5">
+            <div className="flex items-center gap-2.5 border-b border-neutral-100 pb-3">
+              <Bell className="h-4 w-4 text-neutral-500" />
+              <h2 className="text-sm font-semibold text-neutral-900">
+                Benachrichtigungen
+              </h2>
+            </div>
+            <div className="mt-4">
+              <NotificationSettings />
+            </div>
+          </section>
+
           {/* CalDAV Settings */}
           <section className="rounded-xl border border-neutral-200 bg-white p-5">
             <div className="flex items-center gap-2.5 border-b border-neutral-100 pb-3">

frontend/src/components/layout/Header.tsx

@@ -2,6 +2,7 @@
 
 import { createClient } from "@/lib/supabase/client";
 import { TenantSwitcher } from "./TenantSwitcher";
+import { NotificationBell } from "@/components/notifications/NotificationBell";
 import { LogOut } from "lucide-react";
 import { useRouter } from "next/navigation";
 import { useEffect, useState } from "react";
@@ -29,6 +30,7 @@ export function Header() {
       <div className="w-8 lg:w-0" />
       <div className="flex items-center gap-2 sm:gap-3">
         <TenantSwitcher />
+        <NotificationBell />
         {email && (
           <span className="hidden text-sm text-neutral-500 sm:inline">
             {email}

frontend/src/components/notifications/NotificationBell.tsx

@@ -0,0 +1,205 @@
+"use client";
+
+import { useEffect, useRef, useState } from "react";
+import { useQuery, useMutation, useQueryClient } from "@tanstack/react-query";
+import { Bell, Check, CheckCheck, ExternalLink } from "lucide-react";
+import { api } from "@/lib/api";
+import type { Notification, NotificationListResponse } from "@/lib/types";
+
+function getEntityLink(n: Notification): string | null {
+  if (!n.entity_type || !n.entity_id) return null;
+  switch (n.entity_type) {
+    case "deadline":
+      return `/fristen/${n.entity_id}`;
+    case "appointment":
+      return `/termine/${n.entity_id}`;
+    case "case":
+      return `/akten/${n.entity_id}`;
+    default:
+      return null;
+  }
+}
+
+function getTypeColor(type: Notification["type"]): string {
+  switch (type) {
+    case "deadline_overdue":
+      return "bg-red-500";
+    case "deadline_reminder":
+      return "bg-amber-500";
+    case "case_update":
+      return "bg-blue-500";
+    case "assignment":
+      return "bg-violet-500";
+    default:
+      return "bg-neutral-500";
+  }
+}
+
+function timeAgo(dateStr: string): string {
+  const now = new Date();
+  const date = new Date(dateStr);
+  const diffMs = now.getTime() - date.getTime();
+  const diffMin = Math.floor(diffMs / 60000);
+  if (diffMin < 1) return "gerade eben";
+  if (diffMin < 60) return `vor ${diffMin} Min.`;
+  const diffHours = Math.floor(diffMin / 60);
+  if (diffHours < 24) return `vor ${diffHours} Std.`;
+  const diffDays = Math.floor(diffHours / 24);
+  if (diffDays === 1) return "gestern";
+  return `vor ${diffDays} Tagen`;
+}
+
+export function NotificationBell() {
+  const [open, setOpen] = useState(false);
+  const panelRef = useRef<HTMLDivElement>(null);
+  const queryClient = useQueryClient();
+
+  const { data: unreadData } = useQuery({
+    queryKey: ["notifications-unread-count"],
+    queryFn: () =>
+      api.get<{ unread_count: number }>("/api/notifications/unread-count"),
+    refetchInterval: 30_000,
+  });
+
+  const { data: notifData } = useQuery({
+    queryKey: ["notifications"],
+    queryFn: () =>
+      api.get<NotificationListResponse>("/api/notifications?limit=20"),
+    enabled: open,
+  });
+
+  const markRead = useMutation({
+    mutationFn: (id: string) =>
+      api.patch(`/api/notifications/${id}/read`),
+    onSuccess: () => {
+      queryClient.invalidateQueries({ queryKey: ["notifications"] });
+      queryClient.invalidateQueries({
+        queryKey: ["notifications-unread-count"],
+      });
+    },
+  });
+
+  const markAllRead = useMutation({
+    mutationFn: () => api.patch("/api/notifications/read-all"),
+    onSuccess: () => {
+      queryClient.invalidateQueries({ queryKey: ["notifications"] });
+      queryClient.invalidateQueries({
+        queryKey: ["notifications-unread-count"],
+      });
+    },
+  });
+
+  // Close on click outside
+  useEffect(() => {
+    function handleClickOutside(e: MouseEvent) {
+      if (panelRef.current && !panelRef.current.contains(e.target as Node)) {
+        setOpen(false);
+      }
+    }
+    if (open) {
+      document.addEventListener("mousedown", handleClickOutside);
+    }
+    return () => document.removeEventListener("mousedown", handleClickOutside);
+  }, [open]);
+
+  const unreadCount = unreadData?.unread_count ?? 0;
+  const notifications = notifData?.data ?? [];
+
+  return (
+    <div className="relative" ref={panelRef}>
+      <button
+        onClick={() => setOpen(!open)}
+        className="relative rounded-md p-1.5 text-neutral-400 transition-colors hover:bg-neutral-100 hover:text-neutral-600"
+        title="Benachrichtigungen"
+      >
+        <Bell className="h-4 w-4" />
+        {unreadCount > 0 && (
+          <span className="absolute -right-0.5 -top-0.5 flex h-4 min-w-4 items-center justify-center rounded-full bg-red-500 px-1 text-[10px] font-bold text-white">
+            {unreadCount > 99 ? "99+" : unreadCount}
+          </span>
+        )}
+      </button>
+
+      {open && (
+        <div className="absolute right-0 top-full z-50 mt-2 w-80 rounded-xl border border-neutral-200 bg-white shadow-lg sm:w-96">
+          {/* Header */}
+          <div className="flex items-center justify-between border-b border-neutral-100 px-4 py-3">
+            <h3 className="text-sm font-semibold text-neutral-900">
+              Benachrichtigungen
+            </h3>
+            {unreadCount > 0 && (
+              <button
+                onClick={() => markAllRead.mutate()}
+                className="flex items-center gap-1 text-xs text-neutral-500 hover:text-neutral-700"
+              >
+                <CheckCheck className="h-3 w-3" />
+                Alle gelesen
+              </button>
+            )}
+          </div>
+
+          {/* Notification list */}
+          <div className="max-h-96 overflow-y-auto">
+            {notifications.length === 0 ? (
+              <div className="p-6 text-center text-sm text-neutral-400">
+                Keine Benachrichtigungen
+              </div>
+            ) : (
+              notifications.map((n) => {
+                const link = getEntityLink(n);
+                return (
+                  <div
+                    key={n.id}
+                    className={`flex items-start gap-3 border-b border-neutral-50 px-4 py-3 transition-colors last:border-0 ${
+                      n.read_at
+                        ? "bg-white"
+                        : "bg-blue-50/50"
+                    }`}
+                  >
+                    <div
+                      className={`mt-1.5 h-2 w-2 flex-shrink-0 rounded-full ${getTypeColor(n.type)}`}
+                    />
+                    <div className="min-w-0 flex-1">
+                      <p className="text-sm font-medium text-neutral-900 leading-snug">
+                        {n.title}
+                      </p>
+                      {n.body && (
+                        <p className="mt-0.5 text-xs text-neutral-500 line-clamp-2">
+                          {n.body}
+                        </p>
+                      )}
+                      <div className="mt-1.5 flex items-center gap-2">
+                        <span className="text-[11px] text-neutral-400">
+                          {timeAgo(n.created_at)}
+                        </span>
+                        {link && (
+                          <a
+                            href={link}
+                            onClick={() => setOpen(false)}
+                            className="flex items-center gap-0.5 text-[11px] text-blue-600 hover:text-blue-700"
+                          >
+                            <ExternalLink className="h-2.5 w-2.5" />
+                            Anzeigen
+                          </a>
+                        )}
+                      </div>
+                    </div>
+                    {!n.read_at && (
+                      <button
+                        onClick={() => markRead.mutate(n.id)}
+                        className="flex-shrink-0 rounded p-1 text-neutral-400 hover:bg-neutral-100 hover:text-neutral-600"
+                        title="Als gelesen markieren"
+                      >
+                        <Check className="h-3 w-3" />
+                      </button>
+                    )}
+                  </div>
+                );
+              })
+            )}
+          </div>
+        </div>
+      )}
+    </div>
+  );
+}

frontend/src/components/settings/NotificationSettings.tsx

@@ -0,0 +1,167 @@
+"use client";
+
+import { useState } from "react";
+import { useQuery, useMutation, useQueryClient } from "@tanstack/react-query";
+import { api } from "@/lib/api";
+import type { NotificationPreferences } from "@/lib/types";
+
+const REMINDER_OPTIONS = [
+  { value: 14, label: "14 Tage" },
+  { value: 7, label: "7 Tage" },
+  { value: 3, label: "3 Tage" },
+  { value: 1, label: "1 Tag" },
+];
+
+export function NotificationSettings() {
+  const queryClient = useQueryClient();
+  const [saved, setSaved] = useState(false);
+
+  const { data: prefs, isLoading } = useQuery({
+    queryKey: ["notification-preferences"],
+    queryFn: () =>
+      api.get<NotificationPreferences>("/api/notification-preferences"),
+  });
+
+  const [reminderDays, setReminderDays] = useState<number[]>([]);
+  const [emailEnabled, setEmailEnabled] = useState(true);
+  const [dailyDigest, setDailyDigest] = useState(false);
+  const [initialized, setInitialized] = useState(false);
+
+  // Sync state from server once loaded
+  if (prefs && !initialized) {
+    setReminderDays(prefs.deadline_reminder_days);
+    setEmailEnabled(prefs.email_enabled);
+    setDailyDigest(prefs.daily_digest);
+    setInitialized(true);
+  }
+
+  const update = useMutation({
+    mutationFn: (input: {
+      deadline_reminder_days: number[];
+      email_enabled: boolean;
+      daily_digest: boolean;
+    }) => api.put<NotificationPreferences>("/api/notification-preferences", input),
+    onSuccess: () => {
+      queryClient.invalidateQueries({
+        queryKey: ["notification-preferences"],
+      });
+      setSaved(true);
+      setTimeout(() => setSaved(false), 2000);
+    },
+  });
+
+  function toggleDay(day: number) {
+    setReminderDays((prev) =>
+      prev.includes(day) ? prev.filter((d) => d !== day) : [...prev, day].sort((a, b) => b - a),
+    );
+  }
+
+  function handleSave() {
+    update.mutate({
+      deadline_reminder_days: reminderDays,
+      email_enabled: emailEnabled,
+      daily_digest: dailyDigest,
+    });
+  }
+
+  if (isLoading) {
+    return (
+      <div className="animate-pulse space-y-3">
+        <div className="h-4 w-48 rounded bg-neutral-200" />
+        <div className="h-8 w-full rounded bg-neutral-100" />
+        <div className="h-8 w-full rounded bg-neutral-100" />
+      </div>
+    );
+  }
+
+  return (
+    <div className="space-y-5">
+      {/* Reminder days */}
+      <div>
+        <p className="text-sm font-medium text-neutral-700">
+          Fristen-Erinnerungen
+        </p>
+        <p className="mt-0.5 text-xs text-neutral-500">
+          Erinnern Sie mich vor Fristablauf:
+        </p>
+        <div className="mt-2 flex flex-wrap gap-2">
+          {REMINDER_OPTIONS.map((opt) => (
+            <button
+              key={opt.value}
+              onClick={() => toggleDay(opt.value)}
+              className={`rounded-lg border px-3 py-1.5 text-sm transition-colors ${
+                reminderDays.includes(opt.value)
+                  ? "border-blue-500 bg-blue-50 text-blue-700"
+                  : "border-neutral-200 bg-white text-neutral-600 hover:border-neutral-300"
+              }`}
+            >
+              {opt.label}
+            </button>
+          ))}
+        </div>
+      </div>
+
+      {/* Email toggle */}
+      <label className="flex items-center justify-between">
+        <div>
+          <p className="text-sm font-medium text-neutral-700">
+            E-Mail-Benachrichtigungen
+          </p>
+          <p className="text-xs text-neutral-500">
+            Erinnerungen per E-Mail erhalten
+          </p>
+        </div>
+        <button
+          onClick={() => setEmailEnabled(!emailEnabled)}
+          className={`relative h-6 w-11 rounded-full transition-colors ${
+            emailEnabled ? "bg-blue-500" : "bg-neutral-300"
+          }`}
+        >
+          <span
+            className={`absolute left-0.5 top-0.5 h-5 w-5 rounded-full bg-white shadow transition-transform ${
+              emailEnabled ? "translate-x-5" : "translate-x-0"
+            }`}
+          />
+        </button>
+      </label>
+
+      {/* Daily digest toggle */}
+      <label className="flex items-center justify-between">
+        <div>
+          <p className="text-sm font-medium text-neutral-700">
+            Tagesübersicht
+          </p>
+          <p className="text-xs text-neutral-500">
+            Alle Benachrichtigungen gesammelt um 8:00 Uhr per E-Mail
+          </p>
+        </div>
+        <button
+          onClick={() => setDailyDigest(!dailyDigest)}
+          className={`relative h-6 w-11 rounded-full transition-colors ${
+            dailyDigest ? "bg-blue-500" : "bg-neutral-300"
+          }`}
+        >
+          <span
+            className={`absolute left-0.5 top-0.5 h-5 w-5 rounded-full bg-white shadow transition-transform ${
+              dailyDigest ? "translate-x-5" : "translate-x-0"
+            }`}
+          />
+        </button>
+      </label>
+
+      {/* Save */}
+      <div className="flex items-center gap-3 pt-2">
+        <button
+          onClick={handleSave}
+          disabled={update.isPending}
+          className="rounded-md bg-neutral-900 px-4 py-2 text-sm font-medium text-white hover:bg-neutral-800 disabled:opacity-50"
+        >
+          {update.isPending ? "Speichern..." : "Speichern"}
+        </button>
+        {saved && (
+          <span className="text-sm text-green-600">Gespeichert</span>
+        )}
+      </div>
+    </div>
+  );
+}

frontend/src/lib/types.ts

@@ -189,6 +189,37 @@ export interface Note {
   updated_at: string;
 }
 
+// Notifications
+
+export interface Notification {
+  id: string;
+  tenant_id: string;
+  user_id: string;
+  type: "deadline_reminder" | "deadline_overdue" | "case_update" | "assignment";
+  entity_type?: "deadline" | "appointment" | "case";
+  entity_id?: string;
+  title: string;
+  body?: string;
+  sent_at?: string;
+  read_at?: string;
+  created_at: string;
+}
+
+export interface NotificationPreferences {
+  user_id: string;
+  tenant_id: string;
+  deadline_reminder_days: number[];
+  email_enabled: boolean;
+  daily_digest: boolean;
+  created_at?: string;
+  updated_at?: string;
+}
+
+export interface NotificationListResponse {
+  data: Notification[];
+  total: number;
+}
+
 export interface ApiError {
   error: string;
   status: number;