Fix root cause: config.py default had password 'fitness', DB uses trust auth

The actual bug: config.py default database_url included ':fitness' password.
When Coolify doesn't pass DATABASE_URL env var to the container,
pydantic_settings falls back to this default, which PostgreSQL rejects
because the DB initialized with trust authentication (no password).

Fix: removed password from default database_url in config.py.
Also removed unnecessary custom db/Dockerfile.

docker-compose.yml

@@ -34,8 +34,12 @@ services:
       start_period: 30s
 
   db:
-    build: ./db
+    image: postgres:16-alpine
     restart: unless-stopped
+    environment:
+      - POSTGRES_USER=fitness
+      - POSTGRES_DB=fitness_rewards
+      - POSTGRES_HOST_AUTH_METHOD=trust
     volumes:
       - fitness_db_data:/var/lib/postgresql/data
     healthcheck: