feat: time tracking + billing — hourly rates, time entries, invoices (P1)

Database: time_entries, billing_rates, invoices tables with RLS.
Backend: CRUD services+handlers for time entries, billing rates, invoices.
  - Time entries: list/create/update/delete, summary by case/user/month
  - Billing rates: upsert with auto-close previous, current rate lookup
  - Invoices: create with auto-number (RE-YYYY-NNN), status transitions
    (draft->sent->paid, cancellation), link time entries on invoice create
API: 11 new endpoints under /api/time-entries, /api/billing-rates, /api/invoices
Frontend: Zeiterfassung tab on case detail, /abrechnung overview with filters,
  /abrechnung/rechnungen list+detail with status actions, billing rates settings
Also: resolved merge conflicts between audit-trail and role-based branches,
  added missing types (Notification, AuditLogResponse, NotificationPreferences)

frontend/src/app/(app)/abrechnung/rechnungen/page.tsx

@@ -0,0 +1,118 @@
+"use client";
+
+import { useQuery } from "@tanstack/react-query";
+import { api } from "@/lib/api";
+import type { Invoice } from "@/lib/types";
+import { format } from "date-fns";
+import { de } from "date-fns/locale";
+import { Receipt, Loader2 } from "lucide-react";
+import Link from "next/link";
+import { useState } from "react";
+import { Breadcrumb } from "@/components/layout/Breadcrumb";
+
+const STATUS_BADGE: Record<string, string> = {
+  draft: "bg-neutral-100 text-neutral-600",
+  sent: "bg-blue-50 text-blue-700",
+  paid: "bg-emerald-50 text-emerald-700",
+  cancelled: "bg-red-50 text-red-600",
+};
+
+const STATUS_LABEL: Record<string, string> = {
+  draft: "Entwurf",
+  sent: "Versendet",
+  paid: "Bezahlt",
+  cancelled: "Storniert",
+};
+
+export default function RechnungenPage() {
+  const [statusFilter, setStatusFilter] = useState("");
+
+  const { data, isLoading } = useQuery({
+    queryKey: ["invoices", statusFilter],
+    queryFn: () => {
+      const params = statusFilter ? `?status=${statusFilter}` : "";
+      return api.get<{ invoices: Invoice[] }>(`/invoices${params}`);
+    },
+  });
+
+  const invoices = data?.invoices ?? [];
+
+  return (
+    <div className="animate-fade-in">
+      <Breadcrumb
+        items={[
+          { label: "Dashboard", href: "/dashboard" },
+          { label: "Abrechnung", href: "/abrechnung" },
+          { label: "Rechnungen" },
+        ]}
+      />
+
+      <div className="mt-4 flex items-center justify-between">
+        <h1 className="text-lg font-semibold text-neutral-900">Rechnungen</h1>
+      </div>
+
+      {/* Filters */}
+      <div className="mt-4 flex gap-2">
+        {["", "draft", "sent", "paid", "cancelled"].map((s) => (
+          <button
+            key={s}
+            onClick={() => setStatusFilter(s)}
+            className={`rounded-md px-2.5 py-1 text-xs font-medium transition-colors ${
+              statusFilter === s
+                ? "bg-neutral-900 text-white"
+                : "bg-neutral-100 text-neutral-600 hover:bg-neutral-200"
+            }`}
+          >
+            {s === "" ? "Alle" : STATUS_LABEL[s]}
+          </button>
+        ))}
+      </div>
+
+      {isLoading ? (
+        <div className="flex items-center justify-center py-8">
+          <Loader2 className="h-5 w-5 animate-spin text-neutral-400" />
+        </div>
+      ) : invoices.length === 0 ? (
+        <div className="flex flex-col items-center py-12 text-center">
+          <div className="rounded-xl bg-neutral-100 p-3">
+            <Receipt className="h-5 w-5 text-neutral-400" />
+          </div>
+          <p className="mt-2 text-sm text-neutral-500">
+            Keine Rechnungen vorhanden.
+          </p>
+        </div>
+      ) : (
+        <div className="mt-4 space-y-2">
+          {invoices.map((inv) => (
+            <Link
+              key={inv.id}
+              href={`/abrechnung/rechnungen/${inv.id}`}
+              className="flex items-center justify-between rounded-md border border-neutral-200 bg-white px-4 py-3 transition-colors hover:bg-neutral-50"
+            >
+              <div>
+                <div className="flex items-center gap-2">
+                  <p className="text-sm font-medium text-neutral-900">
+                    {inv.invoice_number}
+                  </p>
+                  <span
+                    className={`rounded-full px-2 py-0.5 text-xs font-medium ${STATUS_BADGE[inv.status]}`}
+                  >
+                    {STATUS_LABEL[inv.status]}
+                  </span>
+                </div>
+                <p className="mt-0.5 text-xs text-neutral-500">
+                  {inv.client_name}
+                  {inv.issued_at &&
+                    ` — ${format(new Date(inv.issued_at), "d. MMM yyyy", { locale: de })}`}
+                </p>
+              </div>
+              <p className="text-sm font-semibold text-neutral-900">
+                {inv.total.toFixed(2)} EUR
+              </p>
+            </Link>
+          ))}
+        </div>
+      )}
+    </div>
+  );
+}