Convert GC to async with polling to prevent proxy timeouts

app/admin_api.py

@@ -907,15 +907,11 @@ def gc_run_now():
     if not gc:
         return _json_error("InvalidRequest", "GC is not enabled", 400)
     payload = request.get_json(silent=True) or {}
-    original_dry_run = gc.dry_run
-    if "dry_run" in payload:
-        gc.dry_run = bool(payload["dry_run"])
-    try:
-        result = gc.run_now()
-    finally:
-        gc.dry_run = original_dry_run
+    started = gc.run_async(dry_run=payload.get("dry_run"))
     logger.info("GC manual run by %s", principal.access_key)
-    return jsonify(result.to_dict())
+    if not started:
+        return _json_error("Conflict", "GC is already in progress", 409)
+    return jsonify({"status": "started"})
 
 
 @admin_api_bp.route("/gc/history", methods=["GET"])