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/worker/init.go

@@ -6,6 +6,7 @@ import (
 	"github.com/food-ai/backend/internal/adapters/kafka"
 	"github.com/food-ai/backend/internal/adapters/openai"
 	"github.com/food-ai/backend/internal/domain/dish"
+	"github.com/food-ai/backend/internal/domain/product"
 	"github.com/food-ai/backend/internal/domain/recognition"
 	"github.com/jackc/pgx/v5/pgxpool"
 	"github.com/kelseyhightower/envconfig"
@@ -28,31 +29,57 @@ func loadConfig() (*workerConfig, error) {
 
 // WorkerApp bundles background services that need lifecycle management.
 type WorkerApp struct {
-	workerPool *recognition.WorkerPool
+	workerPool        *recognition.WorkerPool
+	productWorkerPool *recognition.ProductWorkerPool
 }
 
-// Start launches the worker pool goroutines.
+// Start launches the dish and product worker pool goroutines.
 func (workerApp *WorkerApp) Start(applicationContext context.Context) {
 	workerApp.workerPool.Start(applicationContext)
+	workerApp.productWorkerPool.Start(applicationContext)
 }
 
 func initWorker(workerCfg *workerConfig, pool *pgxpool.Pool) (*WorkerApp, error) {
 	openaiClient := openai.NewClient(workerCfg.OpenAIAPIKey)
+
+	// Dish recognition worker.
 	dishRepository := dish.NewRepository(pool)
 	jobRepository := recognition.NewJobRepository(pool)
 
-	topic := recognition.TopicFree
-	groupID := "dish-recognition-free"
+	dishTopic := recognition.TopicFree
+	dishGroupID := "dish-recognition-free"
 	if workerCfg.WorkerPlan == "paid" {
-		topic = recognition.TopicPaid
-		groupID = "dish-recognition-paid"
+		dishTopic = recognition.TopicPaid
+		dishGroupID = "dish-recognition-paid"
 	}
 
-	consumer, consumerError := kafka.NewConsumer(workerCfg.KafkaBrokers, groupID, topic)
-	if consumerError != nil {
-		return nil, consumerError
+	dishConsumer, dishConsumerError := kafka.NewConsumer(workerCfg.KafkaBrokers, dishGroupID, dishTopic)
+	if dishConsumerError != nil {
+		return nil, dishConsumerError
 	}
 
-	workerPool := recognition.NewWorkerPool(jobRepository, openaiClient, dishRepository, consumer)
-	return &WorkerApp{workerPool: workerPool}, nil
+	workerPool := recognition.NewWorkerPool(jobRepository, openaiClient, dishRepository, dishConsumer)
+
+	// Product recognition worker.
+	productRepository := product.NewRepository(pool)
+	productJobRepository := recognition.NewProductJobRepository(pool)
+
+	productTopic := recognition.ProductTopicFree
+	productGroupID := "product-recognition-free"
+	if workerCfg.WorkerPlan == "paid" {
+		productTopic = recognition.ProductTopicPaid
+		productGroupID = "product-recognition-paid"
+	}
+
+	productConsumer, productConsumerError := kafka.NewConsumer(workerCfg.KafkaBrokers, productGroupID, productTopic)
+	if productConsumerError != nil {
+		return nil, productConsumerError
+	}
+
+	productWorkerPool := recognition.NewProductWorkerPool(productJobRepository, openaiClient, productRepository, productConsumer)
+
+	return &WorkerApp{
+		workerPool:        workerPool,
+		productWorkerPool: productWorkerPool,
+	}, nil
 }