feat(sync): add gRPC file synchronization service and integrate end-to-end

- add proto/services/sync_service.proto and generate Go pb + grpc bindings
- implement SyncServiceServer (streaming file scan + download) with:
  - request deduplication, in-memory cache (TTL), chunked streaming
  - concurrent-safe broadcast to waiters and server-side logging
- register SyncSvr in gRPC server and expose sync client in GrpcClient:
  - add syncClient field, registration and safe getters with reconnection-aware timeouts
- integrate gRPC sync into runner:
  - split syncFiles into syncFilesHTTP (legacy) and syncFilesGRPC
  - Runner now chooses implementation via config flag and performs streaming scan/download
- controller improvements:
  - add semaphore-based rate limiting for sync scan requests with in-flight counters and logs
- misc:
  - add utils.IsSyncGrpcEnabled() config helper
  - improve HTTP sync error diagnostics (Content-Type validation, response previews)
  - update/regenerate many protobuf and gRPC generated files (protoc/protoc-gen-go / protoc-gen-go-grpc version bumps)

core/controllers/sync.go

@@ -15,9 +15,29 @@ import (
 var (
 	syncDownloadSemaphore = semaphore.NewWeighted(utils.GetSyncDownloadMaxConcurrency())
 	syncDownloadInFlight  int64
+	syncScanSemaphore     = semaphore.NewWeighted(10) // Limit concurrent scan requests
+	syncScanInFlight      int64
 )
 
 func GetSyncScan(c *gin.Context) (response *Response[entity.FsFileInfoMap], err error) {
+	ctx := c.Request.Context()
+	if ctx == nil {
+		ctx = context.Background()
+	}
+
+	// Rate limiting for scan requests
+	if err := syncScanSemaphore.Acquire(ctx, 1); err != nil {
+		logger.Warnf("failed to acquire sync scan slot for id=%s path=%s: %v", c.Param("id"), c.Param("path"), err)
+		return GetErrorResponse[entity.FsFileInfoMap](errors.Annotate(err, "server overloaded, please retry"))
+	}
+	current := atomic.AddInt64(&syncScanInFlight, 1)
+	logger.Debugf("sync scan in-flight=%d id=%s path=%s", current, c.Param("id"), c.Param("path"))
+	defer func() {
+		newVal := atomic.AddInt64(&syncScanInFlight, -1)
+		logger.Debugf("sync scan completed in-flight=%d id=%s path=%s", newVal, c.Param("id"), c.Param("path"))
+		syncScanSemaphore.Release(1)
+	}()
+
 	workspacePath := utils.GetWorkspace()
 	dirPath := filepath.Join(workspacePath, c.Param("id"), c.Param("path"))
 	files, err := utils.ScanDirectory(dirPath)