feat(fc2a): add /api/import admin endpoints

trigger enqueues a Celery scan_directory task and returns 202 with the
celery task id (the actual scan happens asynchronously; status surfaces
via /api/import/status and /api/system/stats). list_tasks supports
status filter and cursor pagination. retry-failed resets failed tasks
to queued and re-enqueues. clear-completed deletes finished tasks
older than the supplied age (used by the Settings UI's cleanup control).

mode='deep' is explicitly rejected with 400 until FC-2d ships it; the
frontend's UI in FC-2a only sends 'quick'.

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

tests/test_api_import_admin.py

@@ -0,0 +1,90 @@
+from datetime import datetime, timedelta, timezone
+
+import pytest
+
+from backend.app import create_app
+from backend.app.celery_app import celery
+from backend.app.models import ImportBatch, ImportTask
+
+
+@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_rejects_unknown_mode(client):
+    resp = await client.post("/api/import/trigger", json={"mode": "deep"})
+    assert resp.status_code == 400
+
+
+@pytest.mark.asyncio
+async def test_status_when_idle(client):
+    resp = await client.get("/api/import/status")
+    body = await resp.get_json()
+    assert body["active_batch"] is None
+
+
+@pytest.mark.asyncio
+async def test_list_tasks_with_status_filter(client, db):
+    batch = ImportBatch(triggered_by="manual", source_path="/import", scan_mode="quick")
+    db.add(batch)
+    await db.flush()
+    for status in ("complete", "failed", "complete"):
+        db.add(ImportTask(
+            batch_id=batch.id, source_path="/x", task_type="media", status=status,
+            finished_at=datetime.now(timezone.utc),
+        ))
+    await db.commit()
+
+    resp = await client.get("/api/import/tasks?status=failed")
+    body = await resp.get_json()
+    assert len(body["tasks"]) == 1
+    assert body["tasks"][0]["status"] == "failed"
+
+
+@pytest.mark.asyncio
+async def test_retry_failed(client, db):
+    batch = ImportBatch(triggered_by="manual", source_path="/import", scan_mode="quick")
+    db.add(batch)
+    await db.flush()
+    db.add(ImportTask(batch_id=batch.id, source_path="/x", task_type="media", status="failed"))
+    await db.commit()
+
+    resp = await client.post("/api/import/retry-failed")
+    body = await resp.get_json()
+    assert body["retried"] == 1
+
+
+@pytest.mark.asyncio
+async def test_clear_completed(client, db):
+    batch = ImportBatch(triggered_by="manual", source_path="/import", scan_mode="quick")
+    db.add(batch)
+    await db.flush()
+    long_ago = datetime.now(timezone.utc) - timedelta(days=30)
+    db.add(ImportTask(
+        batch_id=batch.id, source_path="/x", task_type="media",
+        status="complete", finished_at=long_ago,
+    ))
+    db.add(ImportTask(
+        batch_id=batch.id, source_path="/y", task_type="media",
+        status="complete", finished_at=datetime.now(timezone.utc),
+    ))
+    await db.commit()
+
+    resp = await client.post("/api/import/clear-completed", json={"age_days": 7})
+    body = await resp.get_json()
+    assert body["deleted"] == 1