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/migrations/002_product_recognition_jobs.sql

@@ -0,0 +1,24 @@
+-- +goose Up
+CREATE TABLE product_recognition_jobs (
+    id           UUID        PRIMARY KEY DEFAULT uuid_generate_v7(),
+    user_id      UUID        NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    user_plan    TEXT        NOT NULL,
+    job_type     TEXT        NOT NULL CHECK (job_type IN ('receipt', 'products')),
+    images       JSONB       NOT NULL,
+    lang         TEXT        NOT NULL DEFAULT 'en',
+    status       TEXT        NOT NULL DEFAULT 'pending',
+    result       JSONB,
+    error        TEXT,
+    created_at   TIMESTAMPTZ NOT NULL DEFAULT now(),
+    started_at   TIMESTAMPTZ,
+    completed_at TIMESTAMPTZ
+);
+
+CREATE INDEX idx_product_recognition_jobs_user
+    ON product_recognition_jobs (user_id, created_at DESC);
+
+CREATE INDEX idx_product_recognition_jobs_status
+    ON product_recognition_jobs (status, user_plan, created_at ASC);
+
+-- +goose Down
+DROP TABLE IF EXISTS product_recognition_jobs;