feat: configure Celery with per-feature queue lanes and a smoke task

Routes are pre-declared for FC-2/FC-3 task modules (import, ml, thumbnail,
download, scan, maintenance). Queue lanes match the ImageRepo pattern where
beat+maintenance run on a separate worker so long imports don't starve
periodic tasks. Smoke ping task confirms the wiring in eager mode for CI.

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

backend/app/tasks/smoke.py

@@ -0,0 +1,8 @@
+"""Smoke task used during FC-1 to confirm Celery routing works end-to-end."""
+
+from ..celery_app import celery
+
+
+@celery.task(name="backend.app.tasks.smoke.ping")
+def ping(value: str = "pong") -> str:
+    return value