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>

backend/cmd/server/app.go

@@ -9,8 +9,9 @@ import (
 
 // App bundles the HTTP handler with background services that need lifecycle management.
 type App struct {
-	handler   http.Handler
-	sseBroker *recognition.SSEBroker
+	handler          http.Handler
+	sseBroker        *recognition.SSEBroker
+	productSSEBroker *recognition.ProductSSEBroker
 }
 
 // ServeHTTP implements http.Handler.
@@ -18,8 +19,9 @@ func (application *App) ServeHTTP(responseWriter http.ResponseWriter, request *h
 	application.handler.ServeHTTP(responseWriter, request)
 }
 
-// Start launches the SSE broker's LISTEN loop.
+// Start launches the SSE brokers' LISTEN loops.
 // Call this once before the HTTP server begins accepting connections.
 func (application *App) Start(applicationContext context.Context) {
 	application.sseBroker.Start(applicationContext)
+	application.productSSEBroker.Start(applicationContext)
 }

backend/cmd/server/init.go

@@ -56,7 +56,9 @@ func initApp(appConfig *config.Config, pool *pgxpool.Pool) (*App, error) {
 	// Recognition pipeline
 	jobRepository := recognition.NewJobRepository(pool)
 	sseBroker := recognition.NewSSEBroker(pool, jobRepository)
-	recognitionHandler := recognition.NewHandler(openaiClient, productRepository, jobRepository, kafkaProducer, sseBroker)
+	productJobRepository := recognition.NewProductJobRepository(pool)
+	productSSEBroker := recognition.NewProductSSEBroker(pool, productJobRepository)
+	recognitionHandler := recognition.NewHandler(openaiClient, productRepository, jobRepository, productJobRepository, kafkaProducer, sseBroker, productSSEBroker)
 
 	menuRepository := menu.NewRepository(pool)
 	menuHandler := menu.NewHandler(menuRepository, openaiClient, openaiClient, dishRepository, pexelsClient, userRepository, userProductRepository, dishRepository)
@@ -93,7 +95,8 @@ func initApp(appConfig *config.Config, pool *pgxpool.Pool) (*App, error) {
 		mainTagListHandler,
 	)
 	return &App{
-		handler:   httpHandler,
-		sseBroker: sseBroker,
+		handler:          httpHandler,
+		sseBroker:        sseBroker,
+		productSSEBroker: productSSEBroker,
 	}, nil
 }