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>

tests/test_celery_smoke.py

@@ -0,0 +1,24 @@
+"""Smoke test — celery runs the ping task in eager mode (no broker required)."""
+
+import pytest
+
+from backend.app.celery_app import celery
+from backend.app.tasks.smoke import ping
+
+
+@pytest.fixture(autouse=True)
+def celery_eager():
+    celery.conf.task_always_eager = True
+    celery.conf.task_eager_propagates = True
+    yield
+    celery.conf.task_always_eager = False
+
+
+def test_ping_returns_pong():
+    result = ping.delay()
+    assert result.get(timeout=1) == "pong"
+
+
+def test_ping_with_value():
+    result = ping.delay("hello")
+    assert result.get(timeout=1) == "hello"