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>

.gitattributes-github

@@ -0,0 +1,12 @@
+# Files to exclude from GitHub mirror
+.env* export-ignore
+docker-compose.prod.yml export-ignore
+.claudedocs/ export-ignore
+backend/data/ export-ignore
+backend/storage/ export-ignore
+backend/.env* export-ignore
+frontend/.env* export-ignore
+secrets/ export-ignore
+*.key export-ignore
+*.pem export-ignore
+.gitea/ export-ignore