feat(F-G-H-I): STL cache, invoices, import validation, notification settings

Phase F — STL Hash Cache:
- Migration 041: step_file_hash column on cad_files
- cache_service.py: SHA256 hash + MinIO-backed STL cache (check/store)
- render_step_thumbnail: compute+persist hash before render
- generate_stl_cache: check MinIO cache before cadquery conversion, store after

Phase G — Invoices:
- Migration 042: invoices + invoice_lines tables with RLS
- Invoice/InvoiceLine models + schemas
- billing service: generate_invoice_number (INV-YYYY-NNNN), create/list/get/delete/PDF
- WeasyPrint PDF generation; backend Dockerfile + pyproject.toml deps
- invoice_router with 6 endpoints; registered in main.py
- frontend: Billing.tsx page + api/billing.ts; route + nav link

Phase H — Import Sanity Check:
- Migration 043: import_validations table
- ImportValidation model + schemas
- run_sanity_check: material fuzzy-match (cutoff=0.8), STEP availability, duplicate detection
- validate_excel_import Celery task (queue: step_processing)
- uploads.py: create ImportValidation on /excel, fire task, expose GET /validations/{id}
- frontend: Upload.tsx polling ValidationDialog with Ampel status indicators

Phase I — Notification Settings:
- Migration 044: notification_configs table (user×event×channel toggles)
- NotificationConfig model + seeds (in_app=true, email=false)
- get/upsert/reset config endpoints on /notifications/config
- frontend: NotificationSettings.tsx page + api/notifications.ts extensions

Infrastructure:
- docker-compose.yml: add worker-thumbnail service (concurrency=1, Q=thumbnail_rendering)
- Fix Dockerfile: libgdk-pixbuf-2.0-0 (correct Debian bookworm package name)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/App.tsx

@@ -15,10 +15,12 @@ import ProductLibraryPage from './pages/ProductLibrary'
 import ProductDetailPage from './pages/ProductDetail'
 import NewProductOrderPage from './pages/NewProductOrder'
 import NotificationsPage from './pages/Notifications'
+import NotificationSettingsPage from './pages/NotificationSettings'
 import PreferencesPage from './pages/Preferences'
 import TenantsPage from './pages/Tenants'
 import WorkflowEditorPage from './pages/WorkflowEditor'
 import MediaBrowserPage from './pages/MediaBrowser'
+import BillingPage from './pages/Billing'
 
 function ProtectedRoute({ children }: { children: React.ReactNode }) {
   const token = useAuthStore((s) => s.token)
@@ -81,6 +83,7 @@ export default function App() {
           <Route path="products" element={<ProductLibraryPage />} />
           <Route path="products/:id" element={<ProductDetailPage />} />
           <Route path="notifications" element={<NotificationsPage />} />
+          <Route path="notification-settings" element={<NotificationSettingsPage />} />
           <Route path="preferences" element={<PreferencesPage />} />
           <Route path="cad/:id" element={<CadPreviewPage />} />
           <Route
@@ -91,6 +94,14 @@ export default function App() {
               </AdminRoute>
             }
           />
+          <Route
+            path="billing"
+            element={
+              <AdminRoute>
+                <BillingPage />
+              </AdminRoute>
+            }
+          />
         </Route>
       </Routes>
     </BrowserRouter>