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

.env.example

@@ -0,0 +1,10 @@
+# Dostalgia — Environment Configuration
+# Copy this file to .env and fill in your values:
+#   cp .env.example .env
+
+# Twitch Client ID for IGDB metadata auto-scrape.
+# Get yours at https://dev.twitch.tv/console/apps → Create Application
+# (Set OAuth Redirect URL to http://localhost, Category to "Other")
+# Optional — IGDB features gracefully skip if not set.
+# TWITCH_CLIENT_ID=your_client_id_here
+# TWITCH_CLIENT_SECRET=your_client_secret_here