feat: async product/receipt recognition via Kafka

Backend:
- Migration 002: product_recognition_jobs table with JSONB images column
  and job_type CHECK ('receipt' | 'products')
- New Kafka topics: ai.products.paid / ai.products.free
- ProductJob model, ProductJobRepository (mirrors dish job pattern)
- itemEnricher extracted from Handler — shared by HTTP handler and worker
- ProductSSEBroker: PG LISTEN on product_job_update channel
- ProductWorkerPool: 5 workers, branches on job_type to call
  RecognizeReceipt or RecognizeProducts per image in parallel
- Handler: RecognizeReceipt and RecognizeProducts now return 202 Accepted
  instead of blocking; 4 new endpoints: GET /ai/product-jobs,
  /product-jobs/history, /product-jobs/{id}, /product-jobs/{id}/stream
- cmd/worker: extended to run ProductWorkerPool alongside dish WorkerPool
- cmd/server: wires productJobRepository + productSSEBroker; both SSE
  brokers started in App.Start()

Flutter client:
- ProductJobCreated, ProductJobResult, ProductJobSummary, ProductJobEvent
  models + submitReceiptRecognition/submitProductsRecognition/stream methods
- Shared _openSseStream helper eliminates duplicate SSE parsing loop
- ScanScreen: replace blocking AI calls with async submit + navigate to
  ProductJobWatchScreen
- ProductJobWatchScreen: watches SSE stream, navigates to /scan/confirm
  when done, shows error on failure
- ProductsScreen: prepends _RecentScansSection (hidden when empty); compact
  horizontal list of recent scans with "See all" → history
- ProductJobHistoryScreen: full list of all product recognition jobs
- New routes: /scan/product-job-watch, /products/job-history
- L10n: 7 new keys in all 12 ARB files

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

client/lib/core/router/app_router.dart

@@ -13,6 +13,8 @@ import '../../features/profile/profile_provider.dart';
 import '../../shared/models/user.dart';
 import '../../features/products/products_screen.dart';
 import '../../features/products/add_product_screen.dart';
+import '../../features/products/product_job_history_screen.dart';
+import '../../features/scan/product_job_watch_screen.dart';
 import '../../features/scan/scan_screen.dart';
 import '../../features/scan/recognition_confirm_screen.dart';
 import '../../features/scan/recognition_service.dart';
@@ -129,6 +131,17 @@ final routerProvider = Provider<GoRouter>((ref) {
         path: '/scan/history',
         builder: (_, __) => const RecognitionHistoryScreen(),
       ),
+      GoRoute(
+        path: '/scan/product-job-watch',
+        builder: (context, state) {
+          final jobCreated = state.extra as ProductJobCreated;
+          return ProductJobWatchScreen(jobCreated: jobCreated);
+        },
+      ),
+      GoRoute(
+        path: '/products/job-history',
+        builder: (_, __) => const ProductJobHistoryScreen(),
+      ),
       ShellRoute(
         builder: (context, state, child) => MainShell(child: child),
         routes: [