feat: add Gitea workflows for selective GitHub mirroring

- Add three different approaches for mirroring to GitHub
- Approach 1: Filter out sensitive files on a separate branch
- Approach 2: Use git archive with .gitattributes exclusions
- Approach 3: Use rsync for flexible file filtering
- Add exclusion lists for sensitive files and directories
- Protect production configs, environment files, and private data

This allows maintaining a public GitHub mirror while keeping
sensitive configuration and data private on the Gitea instance.

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

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

.github-mirror-exclude

@@ -0,0 +1,20 @@
+# Exclude patterns for GitHub mirror
+.env
+.env.*
+.env*
+docker-compose.prod.yml
+docker-compose.traefik.yml
+.claudedocs/
+backend/data/
+backend/storage/
+backend/.env*
+frontend/.env*
+secrets/
+*.key
+*.pem
+.gitea/
+node_modules/
+dist/
+build/
+*.log
+.DS_Store