feat(thumb-backfill): /api/thumbnails/backfill endpoint — POST → 202 + celery_task_id

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

tests/test_api_thumbnails.py

@@ -0,0 +1,32 @@
+import pytest
+
+from backend.app import create_app
+from backend.app.celery_app import celery
+
+pytestmark = pytest.mark.integration
+
+
+@pytest.fixture(autouse=True)
+def eager():
+    celery.conf.task_always_eager = True
+    yield
+    celery.conf.task_always_eager = False
+
+
+@pytest.fixture
+async def app():
+    return create_app()
+
+
+@pytest.fixture
+async def client(app):
+    async with app.test_client() as c:
+        yield c
+
+
+@pytest.mark.asyncio
+async def test_trigger_thumbnail_backfill(client):
+    r = await client.post("/api/thumbnails/backfill")
+    assert r.status_code == 202
+    body = await r.get_json()
+    assert "celery_task_id" in body