feat: implement Iteration 0 foundation (backend + Flutter client)

Backend (Go):
- Project structure with chi router, pgxpool, goose migrations
- JWT auth (access/refresh tokens) with Firebase token verification
- NoopTokenVerifier for local dev without Firebase credentials
- PostgreSQL user repository with atomic profile updates (transactions)
- Mifflin-St Jeor calorie calculation based on profile data
- REST API: POST /auth/login, /auth/refresh, /auth/logout, GET/PUT /profile, GET /health
- Middleware: auth, CORS (localhost wildcard), logging, recovery, request_id
- Unit tests (51 passing) and integration tests (testcontainers)
- Docker Compose setup with postgres healthcheck and graceful shutdown

Flutter client:
- Riverpod state management with GoRouter navigation
- Firebase Auth (email/password + Google sign-in with web popup support)
- Platform-aware API URLs (web/Android/iOS)
- Dio HTTP client with JWT auth interceptor and concurrent refresh handling
- Secure token storage
- Screens: Login, Register, Home (tabs: Menu, Recipes, Products, Profile)
- Unit tests (17 passing)

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

backend/docker-compose.yml

@@ -0,0 +1,36 @@
+services:
+  postgres:
+    image: postgres:16-alpine
+    environment:
+      POSTGRES_DB: food_ai
+      POSTGRES_USER: food_ai
+      POSTGRES_PASSWORD: food_ai_local
+    ports:
+      - "5432:5432"
+    volumes:
+      - pgdata:/var/lib/postgresql/data
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U food_ai -d food_ai"]
+      interval: 5s
+      timeout: 5s
+      retries: 10
+
+  app:
+    build:
+      context: .
+      dockerfile: Dockerfile
+    ports:
+      - "9090:8080"
+    environment:
+      DATABASE_URL: postgres://food_ai:food_ai_local@postgres:5432/food_ai?sslmode=disable
+      FIREBASE_CREDENTIALS_FILE: /app/firebase-credentials.json
+      JWT_SECRET: local-dev-secret-change-in-prod
+      ALLOWED_ORIGINS: http://localhost:3000,http://localhost:9090
+    depends_on:
+      postgres:
+        condition: service_healthy
+    volumes:
+      - ./firebase-credentials.json:/app/firebase-credentials.json:ro
+
+volumes:
+  pgdata: