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/utils/config.go

@@ -348,3 +348,7 @@ func GetMinFileDescriptorLimit() uint64 {
 	}
 	return DefaultMinFileDescriptorLimit
 }
+
+func IsSyncGrpcEnabled() bool {
+	return viper.GetBool("sync.useGrpc")
+}