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>

package.json

@@ -1,7 +1,7 @@
 {
   "name": "gitea-mirror",
   "type": "module",
-  "version": "3.2.2",
+  "version": "3.2.3",
   "engines": {
     "bun": ">=1.2.9"
   },