fix: Complete Issue #72 - Fix automatic mirroring and repository cleanup

Major fixes for Docker environment variable issues and cleanup functionality:

🔧 **Duration Parser & Scheduler Fixes**
- Add comprehensive duration parser supporting "8h", "30m", "24h" formats
- Fix GITEA_MIRROR_INTERVAL environment variable mapping to scheduler
- Auto-enable scheduler when GITEA_MIRROR_INTERVAL is set
- Improve scheduler logging to clarify timing behavior (from last run, not startup)

🧹 **Repository Cleanup Service**
- Complete repository cleanup service for orphaned repos (unstarred, deleted)
- Fix cleanup configuration logic - now works with CLEANUP_DELETE_IF_NOT_IN_GITHUB=true
- Auto-enable cleanup when deleteIfNotInGitHub is enabled
- Add manual cleanup trigger API endpoint (/api/cleanup/trigger)
- Support archive/delete actions with dry-run mode and protected repos

🐛 **Environment Variable Integration**
- Fix scheduler not recognizing GITEA_MIRROR_INTERVAL=8h
- Fix cleanup requiring both CLEANUP_DELETE_FROM_GITEA and CLEANUP_DELETE_IF_NOT_IN_GITHUB
- Auto-enable services when relevant environment variables are set
- Better error logging and debugging information

📚 **Documentation Updates**
- Update .env.example with auto-enabling behavior notes
- Update ENVIRONMENT_VARIABLES.md with clarified functionality
- Add comprehensive tests for duration parsing

This resolves the core issues where:
1. GITEA_MIRROR_INTERVAL=8h was not working for automatic mirroring
2. Repository cleanup was not working despite CLEANUP_DELETE_IF_NOT_IN_GITHUB=true
3. Users had no visibility into why scheduling/cleanup wasn't working

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

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

.env.example

@@ -71,7 +71,7 @@ DOCKER_TAG=latest
 
 # Repository Settings
 # GITEA_ORG_VISIBILITY=public  # Options: public, private, limited, default
-# GITEA_MIRROR_INTERVAL=8h  # Mirror sync interval (e.g., 30m, 1h, 8h, 24h)
+# GITEA_MIRROR_INTERVAL=8h  # Mirror sync interval (e.g., 30m, 1h, 8h, 24h) - automatically enables scheduler
 # GITEA_LFS=false  # Enable LFS support
 # GITEA_CREATE_ORG=true  # Auto-create organizations
 # GITEA_PRESERVE_VISIBILITY=false  # Preserve GitHub repo visibility in Gitea
@@ -150,7 +150,7 @@ DOCKER_TAG=latest
 
 # Repository Cleanup
 # CLEANUP_DELETE_FROM_GITEA=false  # Delete repos from Gitea
-# CLEANUP_DELETE_IF_NOT_IN_GITHUB=true  # Delete if not in GitHub
+# CLEANUP_DELETE_IF_NOT_IN_GITHUB=true  # Delete if not in GitHub - automatically enables cleanup
 # CLEANUP_ORPHANED_REPO_ACTION=archive  # Options: skip, archive, delete
 # CLEANUP_DRY_RUN=true  # Test mode without actual deletion