docs: add IGDB setup guide, .env.example, and .gitignore for public release

- Add .env.example with Twitch credential instructions
- Update docker-compose.yml to use env_file instead of inline comments
- Add IGDB Metadata section to README with setup walkthrough
- Add .env / .env.local to .gitignore to prevent secret leaks

docker-compose.yml

@@ -6,9 +6,8 @@ services:
       - "8765:8765"
     volumes:
       - ./data:/data
+    env_file:
+      - .env
     environment:
       - DOSTALGIA_DATA_DIR=/data
-      # IGDB credentials — set these for auto-scrape to work:
-      # - TWITCH_CLIENT_ID=your_client_id
-      # - TWITCH_CLIENT_SECRET=your_client_secret
     restart: unless-stopped