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/l10n/app_en.arb

@@ -28,7 +28,9 @@
   "queuePosition": "Position {position}",
   "@queuePosition": {
     "placeholders": {
-      "position": { "type": "int" }
+      "position": {
+        "type": "int"
+      }
     }
   },
   "processing": "Processing...",
@@ -116,7 +118,9 @@
   "noResultsForQuery": "Nothing found for \"{query}\"",
   "@noResultsForQuery": {
     "placeholders": {
-      "query": { "type": "String" }
+      "query": {
+        "type": "String"
+      }
     }
   },
   "servingsLabel": "Servings",
@@ -125,7 +129,9 @@
   "planningForDate": "Planning for {date}",
   "@planningForDate": {
     "placeholders": {
-      "date": { "type": "String" }
+      "date": {
+        "type": "String"
+      }
     }
   },
   "markAsEaten": "Mark as eaten",
@@ -134,7 +140,6 @@
   "generateWeekSubtitle": "AI will create a menu with breakfast, lunch and dinner for the whole week",
   "generatingMenu": "Generating menu...",
   "dayPlannedLabel": "Day planned",
-
   "planMenuButton": "Plan meals",
   "planMenuTitle": "What to plan?",
   "planOptionSingleMeal": "Single meal",
@@ -149,16 +154,23 @@
   "planSelectMealType": "Meal type",
   "planSelectRange": "Select period",
   "planGenerateButton": "Plan",
-  "planGenerating": "Generating plan\u2026",
+  "planGenerating": "Generating plan…",
   "planSuccess": "Menu planned!",
   "planProductsTitle": "Products for the menu",
   "planProductsSubtitle": "AI will take the selected products into account when generating recipes",
   "planProductsEmpty": "No products added",
-  "planProductsEmptyMessage": "Add products you have at home \u2014 AI will suggest recipes from what you already have",
+  "planProductsEmptyMessage": "Add products you have at home — AI will suggest recipes from what you already have",
   "planProductsAddProducts": "Add products",
   "planProductsContinue": "Continue",
   "planProductsSkip": "Skip product selection",
   "planProductsSkipNoProducts": "Plan without products",
   "planProductsSelectAll": "Select all",
-  "planProductsDeselectAll": "Deselect all"
+  "planProductsDeselectAll": "Deselect all",
+  "recentScans": "Recent scans",
+  "seeAllScans": "See all",
+  "productJobHistoryTitle": "Scan history",
+  "jobTypeReceipt": "Receipt",
+  "jobTypeProducts": "Products",
+  "scanSubmitting": "Submitting...",
+  "processingProducts": "Processing..."
 }