feat(files): add v2 task and metadata workflows

backend/src/main/java/com/yoyuzh/files/BackgroundTaskHandlerResult.java

@@ -0,0 +1,10 @@
+package com.yoyuzh.files;
+
+import java.util.Map;
+
+public record BackgroundTaskHandlerResult(Map<String, Object> publicStatePatch) {
+
+    public static BackgroundTaskHandlerResult empty() {
+        return new BackgroundTaskHandlerResult(Map.of());
+    }
+}