feat(fc3c): download_source Celery task + register in celery_app

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

tests/test_download_source_task.py

@@ -0,0 +1,14 @@
+"""Smoke test that the Celery task is registered and routed correctly."""
+
+from backend.app.celery_app import celery
+
+
+def test_download_source_is_registered():
+    name = "backend.app.tasks.download.download_source"
+    assert name in celery.tasks
+
+
+def test_download_source_routes_to_download_queue():
+    routes = celery.conf.task_routes
+    assert "backend.app.tasks.download.*" in routes
+    assert routes["backend.app.tasks.download.*"]["queue"] == "download"