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/components/layout/Sidebar.tsx

@@ -11,6 +11,7 @@ import {
   Settings,
   Menu,
   X,
+  Receipt,
 } from "lucide-react";
 import { useState, useEffect } from "react";
 import { usePermissions } from "@/lib/hooks/usePermissions";
@@ -27,6 +28,7 @@ const allNavigation: NavItem[] = [
   { name: "Akten", href: "/cases", icon: FolderOpen },
   { name: "Fristen", href: "/fristen", icon: Clock },
   { name: "Termine", href: "/termine", icon: Calendar },
+  { name: "Abrechnung", href: "/abrechnung", icon: Receipt, permission: "manage_billing" },
   { name: "AI Analyse", href: "/ai/extract", icon: Brain, permission: "ai_extraction" },
   { name: "Einstellungen", href: "/einstellungen", icon: Settings, permission: "manage_settings" },
 ];