feat(fc2a): add /api/import admin endpoints

trigger enqueues a Celery scan_directory task and returns 202 with the
celery task id (the actual scan happens asynchronously; status surfaces
via /api/import/status and /api/system/stats). list_tasks supports
status filter and cursor pagination. retry-failed resets failed tasks
to queued and re-enqueues. clear-completed deletes finished tasks
older than the supplied age (used by the Settings UI's cleanup control).

mode='deep' is explicitly rejected with 400 until FC-2d ships it; the
frontend's UI in FC-2a only sends 'quick'.

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

backend/app/api/__init__.py

@@ -15,6 +15,7 @@ api_bp.add_url_rule("/health", view_func=health.get_health, methods=["GET"])
 
 def all_blueprints() -> list[Blueprint]:
     from .gallery import gallery_bp
+    from .import_admin import import_admin_bp
     from .settings import settings_bp
     from .tags import tags_bp
-    return [api_bp, gallery_bp, tags_bp, settings_bp]
+    return [api_bp, gallery_bp, tags_bp, settings_bp, import_admin_bp]