feat(deep-scan): trigger accepts 'deep'; scan_directory(mode) + enqueue backfill_phash

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

tests/test_api_import_admin.py

@@ -90,3 +90,33 @@ async def test_clear_completed(client, db):
     resp = await client.post("/api/import/clear-completed", json={"age_days": 7})
     body = await resp.get_json()
     assert body["deleted"] == 1
+
+
+@pytest.mark.asyncio
+async def test_trigger_accepts_deep(client, monkeypatch):
+    # Stub the task dispatch — assert the API accepts 'deep' and forwards
+    # mode, without running the real (eager) scan_directory.
+    from backend.app.tasks import scan as scan_mod
+
+    seen = {}
+
+    class _Res:
+        id = "celery-deep"
+
+    def _fake_delay(*, triggered_by, mode):
+        seen["triggered_by"] = triggered_by
+        seen["mode"] = mode
+        return _Res()
+
+    monkeypatch.setattr(scan_mod.scan_directory, "delay", _fake_delay)
+    resp = await client.post("/api/import/trigger", json={"mode": "deep"})
+    assert resp.status_code == 202
+    body = await resp.get_json()
+    assert body["mode"] == "deep"
+    assert seen == {"triggered_by": "manual", "mode": "deep"}
+
+
+@pytest.mark.asyncio
+async def test_trigger_still_rejects_unknown_mode(client):
+    resp = await client.post("/api/import/trigger", json={"mode": "wat"})
+    assert resp.status_code == 400