feat: add build info (commit SHA + deploy timestamp) to health endpoint and admin sidebar

- deploy.sh writes .build-info with commit SHA and timestamp after git pull
- /health endpoint now returns version, commit, and deployed_at fields
- Admin sidebar footer shows version and commit SHA
- Hetzner docs updated: runner --config flag, swap, and runner timeout

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitignore

@@ -161,6 +161,7 @@ docker-compose.override.yml
 *.override.yml
 
 # Deployment & Security
+.build-info
 deployment-local/
 *.pem
 *.key