feat: implement Iteration 4 — menu planning, shopping list, diary

Backend:
- Migrations 007 (menu_plans, menu_items, shopping_lists) and
  008 (meal_diary)
- gemini/menu.go: GenerateMenu — 7-day × 3-meal plan via one Groq call
- internal/menu: model, repository (GetByWeek, SaveMenuInTx,
  shopping list CRUD), handler (GET/PUT/DELETE /menu,
  POST /ai/generate-menu, shopping list endpoints)
- internal/diary: model, repository, handler (GET/POST/DELETE /diary)
- Increase server WriteTimeout to 120s for long AI calls
- api_client.go: add patch() and postList() helpers

Flutter:
- shared/models: menu.dart, shopping_item.dart, diary_entry.dart
- features/menu: menu_service.dart, menu_provider.dart
  (MenuNotifier, ShoppingListNotifier, DiaryNotifier with family)
- MenuScreen: 7-day view, week nav, skeleton on generation,
  generate FAB with confirmation dialog
- ShoppingListScreen: items by category, optimistic checkbox toggle
- DiaryScreen: daily entries with swipe-to-delete, add-entry sheet
- Router: /menu/shopping-list and /menu/diary routes

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/migrations/008_create_meal_diary.sql

@@ -0,0 +1,22 @@
+-- +goose Up
+
+CREATE TABLE meal_diary (
+    id         UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    user_id    UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    date       DATE NOT NULL,
+    meal_type  TEXT NOT NULL,
+    name       TEXT NOT NULL,
+    portions   DECIMAL(5,2) NOT NULL DEFAULT 1,
+    calories   DECIMAL(8,2),
+    protein_g  DECIMAL(8,2),
+    fat_g      DECIMAL(8,2),
+    carbs_g    DECIMAL(8,2),
+    source     TEXT NOT NULL DEFAULT 'manual',
+    recipe_id  UUID REFERENCES saved_recipes(id) ON DELETE SET NULL,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT now()
+);
+
+CREATE INDEX idx_meal_diary_user_date ON meal_diary(user_id, date);
+
+-- +goose Down
+DROP TABLE meal_diary;