Switch PostgreSQL to Alpine-based image for consistency

- Change from postgres:15 to postgres:15-alpine
- Maintains consistency with Alpine-based API container
- Smaller image size and better ARM64 support
- All services now use Alpine or minimal base images

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

docker-compose.yml

@@ -2,7 +2,7 @@ version: '3.8'
 
 services:
   postgres:
-    image: postgres:15
+    image: postgres:15-alpine
     environment:
       POSTGRES_USER: user
       POSTGRES_PASSWORD: password