feat: comprehensive environment variable support

- Added support for 60+ environment variables covering all configuration options
- Created detailed documentation in docs/ENVIRONMENT_VARIABLES.md with tables
- Fixed missing skipStarredIssues field in GitHub config
- Updated docker-compose files to reference environment variable documentation
- Updated README to link to the new environment variables documentation
- Environment variables now populate UI configuration automatically on Docker startup
- Preserves manual UI changes when environment variables are not set
- Includes support for mirror metadata, scheduling, cleanup, and authentication options

Fixes #69

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

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

docker-compose.alt.yml

@@ -11,6 +11,8 @@ services:
     volumes:
       - ./data:/app/data
     environment:
+      # For a complete list of all supported environment variables, see:
+      # docs/ENVIRONMENT_VARIABLES.md or .env.example
       - NODE_ENV=production
       - DATABASE_URL=file:data/gitea-mirror.db
       - HOST=0.0.0.0