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/features/scan/scan_screen.dart

@@ -82,30 +82,26 @@ class _ScanScreenState extends ConsumerState<ScanScreen> {
     final service = ref.read(recognitionServiceProvider);
     final l10n = AppLocalizations.of(context)!;
 
-    // Show loading overlay while the AI processes.
     showDialog(
       context: context,
       barrierDismissible: false,
-      builder: (dialogContext) => _LoadingDialog(label: l10n.recognizing),
+      builder: (dialogContext) => _LoadingDialog(label: l10n.scanSubmitting),
     );
 
     try {
+      ProductJobCreated jobCreated;
       switch (mode) {
         case _Mode.receipt:
-          final result = await service.recognizeReceipt(files.first);
-          if (context.mounted) {
-            Navigator.pop(context); // close loading
-            context.push('/scan/confirm', extra: result.items);
-          }
+          jobCreated = await service.submitReceiptRecognition(files.first);
         case _Mode.products:
-          final items = await service.recognizeProducts(files);
-          if (context.mounted) {
-            Navigator.pop(context);
-            context.push('/scan/confirm', extra: items);
-          }
+          jobCreated = await service.submitProductsRecognition(files);
+      }
+      if (context.mounted) {
+        Navigator.pop(context); // close loading dialog
+        context.push('/scan/product-job-watch', extra: jobCreated);
       }
     } catch (recognitionError) {
-      debugPrint('Recognition error: $recognitionError');
+      debugPrint('Recognition submit error: $recognitionError');
       if (context.mounted) {
         Navigator.pop(context); // close loading
         ScaffoldMessenger.of(context).showSnackBar(