dev: add fs.written event

Unlike fs.write, fs.written is only emitted after the contents of the
fily have been fully streamed and comitted to the delegate storage.

src/backend/src/filesystem/ll_operations/ll_write.js

@@ -217,6 +217,10 @@ class LLOWrite extends LLWriteBase {
             await entryOp.awaitDone();
             this.log.debug('[owrite] finished creating fsentry', { uid })
             resourceService.free(uid);
+            svc_event.emit('fs.written.file', {
+                node,
+                context: this.context,
+            });
         })();
 
         state_upload.post_insert({