feat: add PostgreSQL to production setup (v9b)

Prepares production for SQLite → PostgreSQL migration:
- Add postgres service (mitai-db, port 5432)
- Add DB environment variables to backend
- Backend depends on postgres health check
- Uses startup.sh for automatic migration

Migration strategy:
1. SQLite data in /app/data/bodytrack.db is preserved (volume mounted)
2. On first start with empty PostgreSQL: automatic migration
3. Migration is safe: checks if profiles table is empty before migrating
4. After migration: all new data goes to PostgreSQL

IMPORTANT: Set DB_PASSWORD in .env before deploying!

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

docker-compose.yml

@@ -1,22 +1,55 @@
 services:
+  postgres:
+    image: postgres:16-alpine
+    container_name: mitai-db
+    restart: unless-stopped
+    environment:
+      POSTGRES_DB: mitai_prod
+      POSTGRES_USER: mitai_prod
+      POSTGRES_PASSWORD: ${DB_PASSWORD:-change_me_in_production}
+    volumes:
+      - mitai_postgres_data:/var/lib/postgresql/data
+    ports:
+      - "127.0.0.1:5432:5432"
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U mitai_prod"]
+      interval: 10s
+      timeout: 5s
+      retries: 5
+
   backend:
     build: ./backend
     container_name: mitai-api
     restart: unless-stopped
     ports:
       - "8002:8000"
+    depends_on:
+      postgres:
+        condition: service_healthy
     volumes:
       - bodytrack_bodytrack-data:/app/data
       - bodytrack_bodytrack-photos:/app/photos
     environment:
+      # Database
+      - DB_HOST=postgres
+      - DB_PORT=5432
+      - DB_NAME=mitai_prod
+      - DB_USER=mitai_prod
+      - DB_PASSWORD=${DB_PASSWORD:-change_me_in_production}
+
+      # AI
       - OPENROUTER_API_KEY=${OPENROUTER_API_KEY}
       - OPENROUTER_MODEL=${OPENROUTER_MODEL:-anthropic/claude-sonnet-4}
       - ANTHROPIC_API_KEY=${ANTHROPIC_API_KEY}
+
+      # Email
       - SMTP_HOST=${SMTP_HOST}
       - SMTP_PORT=${SMTP_PORT:-587}
       - SMTP_USER=${SMTP_USER}
       - SMTP_PASS=${SMTP_PASS}
       - SMTP_FROM=${SMTP_FROM}
+
+      # App
       - APP_URL=${APP_URL}
       - DATA_DIR=/app/data
       - PHOTOS_DIR=/app/photos
@@ -33,6 +66,7 @@ services:
       - backend
 
 volumes:
+  mitai_postgres_data:
   bodytrack_bodytrack-data:
     external: true
   bodytrack_bodytrack-photos: