fix(nc-webhook): propagate folder deletes + resurrect un-discarded files

Two bugs surfaced by the Phase 2 deletion-roundtrip test:

A) Folder delete in NC only fires one NodeDeletedEvent (for the folder
   itself, no .jpg suffix). The handler bailed with "unsupported
   extension" and photos inside the folder kept is_discarded=false in
   mule until the 30-min discard_missing_photos_beat caught up.

   Fix: when the deleted path has no supported extension, call new
   `handle_directory_deletion()` which UPDATEs every Photo whose
   filepath starts with `dirpath + '/'`. Single SQL statement,
   idempotent (excludes already-discarded rows so re-deliveries don't
   re-stamp discarded_at).

C) PUT-overwrite of a previously-discarded file fired NodeWrittenEvent
   → scan_folder, but scan_folder's "Photo exists by filepath, skip"
   branch left is_discarded=true. File was back on disk; mule still
   treated it as gone.

   Fix: in that branch, if the existing row is discarded, flip
   is_discarded=false + clear discarded_at + re-queue extract_metadata
   so EXIF / nextcloud_fileid pick up any changes to the bytes.

Together these close the gap for "delete then put back" round-trips
via the NC webhook path. Trashbin-restore (bug B in the test report)
remains an NC-side gap — NC doesn't emit any event mule subscribes to
for restore-from-trash. That stays a TODO.

backend/app/tasks/scan.py

@@ -204,8 +204,25 @@ async def _scan_folder_async(folder_path: str, source_root_id: Optional[str], ta
                             existing = await session.execute(
                                 select(Photo).where(Photo.filepath == filepath)
                             )
-                            if existing.scalar_one_or_none():
-                                logger.debug(f"File already indexed: {filepath}")
+                            existing_photo = existing.scalar_one_or_none()
+                            if existing_photo is not None:
+                                # Resurrect a previously-discarded row if the
+                                # file is back on disk. WebDAV DELETE +
+                                # re-upload, trashbin restore via PUT-overwrite,
+                                # and any "I removed it then put it back" flow
+                                # all land here. Re-queue extract_metadata in
+                                # case the bytes changed (different EXIF, new
+                                # nextcloud_fileid).
+                                if existing_photo.is_discarded:
+                                    existing_photo.is_discarded = False
+                                    existing_photo.discarded_at = None
+                                    await session.commit()
+                                    logger.info(
+                                        f"Resurrected discarded photo on rescan: {filepath}"
+                                    )
+                                    extract_metadata.delay(existing_photo.id)
+                                else:
+                                    logger.debug(f"File already indexed: {filepath}")
                                 processed_files += 1
                                 progress_set(REDIS_KEY_PROCESSED, processed_files)
                                 continue
@@ -506,6 +523,35 @@ async def handle_file_deletion(filepath: str):
             logger.info(f"Marked photo as discarded: {filepath}")
 
 
+async def handle_directory_deletion(dirpath: str) -> int:
+    """Mark every Photo under `dirpath` as discarded — used when Nextcloud
+    fires a NodeDeletedEvent on a folder. NC emits ONE event for the
+    folder itself (not one per child file), so without this we'd never
+    see the children disappear except via the 30-min reconcile sweep.
+
+    Returns the number of photos affected. Matches by `filepath LIKE
+    dirpath + '/%'` (the trailing slash is important — we don't want
+    `/photos/foo` to also match `/photos/foobar.jpg`).
+    """
+    from sqlalchemy import update
+
+    prefix = dirpath.rstrip("/") + "/"
+    async with AsyncSessionLocal() as session:
+        result = await session.execute(
+            update(Photo)
+            .where(
+                Photo.filepath.like(prefix + "%"),
+                Photo.is_discarded.is_(False),
+            )
+            .values(is_discarded=True, discarded_at=datetime.utcnow())
+        )
+        await session.commit()
+        n = result.rowcount or 0
+    if n:
+        logger.info(f"Marked {n} photos as discarded under {dirpath}")
+    return n
+
+
 @shared_task(name='backfill_gps')
 def backfill_gps():
     """Re-run metadata extraction on every non-discarded photo that is