feat(nc-webhook): handle folder rename via NodeRenamedEvent

NC fires one NodeRenamedEvent on a directory rename — children don't
get their own events. The handler bailed on both paths having no
supported extension. Now:

- New `handle_directory_rename(old, new)` in scan.py does a single
  transaction of prefix-rewrites against photos.filepath, folders.path,
  and source_roots.path. Cross-source-root case (Photos/x → Memories/x)
  is treated as discard-the-old-subtree; scan_folder dispatched by the
  subsequent NodeWritten/NodeCreated picks up the new root.

- Webhook renamed branch checks "both source and target are
  directories" and calls the helper. File renames keep the existing
  delete-old + scan-new-parent path.

Idempotent: the SQL matches zero rows the second time around. That
makes the feedback loop safe — mule's existing PATCH /folders/{id}
endpoint already does a WebDAV MOVE + inline DB rewrite for NC paths,
and the resulting NodeRenamedEvent now flows back through this handler
without re-running the rewrite or leaving rows stale.

Trashbin restore (the documented "NC doesn't emit a subscribed event"
gap) is unchanged.

backend/app/routers/nc_webhook.py

@@ -130,6 +130,7 @@ async def nc_webhook(
         scan_folder,
         handle_file_deletion,
         handle_directory_deletion,
+        handle_directory_rename,
     )
 
     supported = _supported_extensions()
@@ -183,9 +184,35 @@ async def nc_webhook(
         target = event.get("target") or {}
         old_abs = _nc_path_to_abs(source.get("path") or "")
         new_abs = _nc_path_to_abs(target.get("path") or "")
-        if old_abs and Path(old_abs).suffix.lower() in supported:
+        if not old_abs or not new_abs:
+            return {"status": "ignored", "reason": "non-user-files path"}
+
+        old_is_dir = Path(old_abs).suffix.lower() not in supported
+        new_is_dir = Path(new_abs).suffix.lower() not in supported
+
+        # Directory rename: NC fires one event for the directory; the
+        # children's paths change implicitly. Prefix-rewrite in mule.
+        # Same handler covers the feedback case where the PATCH
+        # /folders/{id}/rename endpoint already updated the DB — the
+        # SQL UPDATE matches zero rows the second time around.
+        if old_is_dir and new_is_dir:
+            result = await handle_directory_rename(old_abs, new_abs)
+            logger.info(
+                "nc-webhook renamed (dir): %s -> %s : %s",
+                old_abs, new_abs, result,
+            )
+            return {
+                "status": "applied",
+                "action": "rename_subtree",
+                "from": old_abs,
+                "to": new_abs,
+                **result,
+            }
+
+        # File rename (existing logic).
+        if Path(old_abs).suffix.lower() in supported:
             await handle_file_deletion(old_abs)
-        if new_abs and Path(new_abs).suffix.lower() in supported:
+        if Path(new_abs).suffix.lower() in supported:
             parent_dir = str(Path(new_abs).parent)
             source_root_id = await _source_root_for(parent_dir)
             if source_root_id is not None: