refactor: add EC shard count fields to VolumeInfo protobuf

- Add data_shards_count field (field 8) to VolumeInfo message
- Add parity_shards_count field (field 9) to VolumeInfo message
- Fields are optional, 0 means use default (10+4)
- Backward compatible: fields added at end
- Phase 1: Foundation for future customization

weed/pb/volume_server.proto

@@ -525,6 +525,8 @@ message VolumeInfo {
     int64 dat_file_size = 5; // store the original dat file size
     uint64 expire_at_sec = 6; // expiration time of ec volume
     bool read_only = 7;
+    uint32 data_shards_count = 8; // EC data shards (0 = use default 10)
+    uint32 parity_shards_count = 9; // EC parity shards (0 = use default 4)
 }
 message OldVersionVolumeInfo {
     repeated RemoteFile files = 1;