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/celery_app.py

@@ -0,0 +1,45 @@
+"""Celery configuration with separate queue lanes.
+
+Queues:
+  import      — filesystem import path (FC-2)
+  ml          — WD14 + SigLIP inference (FC-2; runs in the ml-worker image)
+  thumbnail   — image and video thumbnail generation (FC-2)
+  download    — gallery-dl tasks (FC-3)
+  scan        — periodic source checks (FC-3) — kept separate so long imports
+                don't starve the scheduler
+  maintenance — pHash recomputation, centroid rebuild, etc. (FC-2/FC-3)
+  default     — anything not explicitly routed
+"""
+
+from celery import Celery
+
+from .config import get_config
+
+
+def make_celery() -> Celery:
+    cfg = get_config()
+    app = Celery(
+        "fabledcurator",
+        broker=cfg.celery_broker_url,
+        backend=cfg.celery_result_backend,
+        include=["backend.app.tasks.smoke"],  # FC-2/FC-3 extend this list
+    )
+    app.conf.update(
+        task_default_queue="default",
+        task_routes={
+            "backend.app.tasks.import_file.*": {"queue": "import"},
+            "backend.app.tasks.ml.*": {"queue": "ml"},
+            "backend.app.tasks.thumbnail.*": {"queue": "thumbnail"},
+            "backend.app.tasks.download.*": {"queue": "download"},
+            "backend.app.tasks.scan.*": {"queue": "scan"},
+            "backend.app.tasks.maintenance.*": {"queue": "maintenance"},
+        },
+        # Heavy ML tasks need fair dispatch — see ImageRepo's precedent.
+        task_acks_late=True,
+        worker_prefetch_multiplier=1,
+        broker_connection_retry_on_startup=True,
+    )
+    return app
+
+
+celery = make_celery()

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