feat(deep-scan): trigger accepts 'deep'; scan_directory(mode) + enqueue backfill_phash

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/tasks/scan.py

@@ -34,8 +34,12 @@ def _sync_session_factory():
 
 
 @celery.task(name="backend.app.tasks.scan.scan_directory", bind=True)
-def scan_directory(self, triggered_by: str = "manual") -> int:
-    """Walks the import root and creates ImportTasks. Returns the batch id."""
+def scan_directory(self, triggered_by: str = "manual",
+                   mode: str = "quick") -> int:
+    """Walks the import root and creates ImportTasks. `mode` is 'quick'
+    or 'deep'; deep also enqueues a library-wide phash backfill and (via
+    Importer.deep) re-derives on already-imported files. Returns the
+    batch id."""
     SessionLocal = _sync_session_factory()
     with SessionLocal() as session:
         settings = session.execute(
@@ -46,7 +50,7 @@ def scan_directory(self, triggered_by: str = "manual") -> int:
         batch = ImportBatch(
             triggered_by=triggered_by,
             source_path=str(import_root),
-            scan_mode="quick",  # FC-2a is quick only; FC-2d adds 'deep'
+            scan_mode=mode,
             status="running",
         )
         session.add(batch)
@@ -84,4 +88,9 @@ def scan_directory(self, triggered_by: str = "manual") -> int:
             import_media_file.delay(task.id)
         session.commit()
 
+        if mode == "deep":
+            from .maintenance import backfill_phash
+
+            backfill_phash.delay()
+
     return batch_id