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>

main.py

@@ -306,10 +306,15 @@ def health_check(db: Session = Depends(get_db)):
     """Health check endpoint"""
     try:
         # Test database connection
+        from app.core.build_info import get_build_info
+        build = get_build_info()
         db.execute(text("SELECT 1"))
         return {
             "status": "healthy",
             "timestamp": datetime.now(UTC),
+            "version": settings.version,
+            "commit": build["commit"],
+            "deployed_at": build["deployed_at"],
             "message": f"{settings.project_name} v{settings.version}",
             "docs": {
                 "swagger": "/docs",