fix: seed admin user on startup and run sync as background task

- Seed/update admin user from ADMIN_USERNAME/ADMIN_PASSWORD settings on
  startup (admin_users table was empty, login was impossible)
- POST /api/admin/sync now returns immediately and runs the equipment
  sync via BackgroundTasks (full sync took minutes and hit nginx 504)
- run_sync accepts optional sync_log_id to reuse a pre-created log entry
- Adapt admin router test to background sync behavior

backend/tests/test_admin_router.py

@@ -35,15 +35,18 @@ async def test_sync_with_valid_token(client, seeded_admin, test_db):
     })
     token = login_resp.json()["access_token"]
 
-    # Mock sync service
+    # Mock sync service (background task)
     with patch.object(SyncService, "run_sync", new_callable=AsyncMock) as mock_sync:
-        mock_sync.return_value = {"sync_id": 42, "items_processed": 10, "items_failed": 0, "status": "completed"}
+        mock_sync.return_value = {"sync_id": 1, "items_processed": 10, "items_failed": 0, "status": "completed"}
         resp = await client.post("/api/admin/sync", cookies={"hms_admin_token": token})
 
     assert resp.status_code == 200
     data = resp.json()
-    assert data["sync_id"] == 42
-    assert data["status"] == "completed"
+    # Background sync: request returns immediately with a running log entry
+    assert data["sync_id"] == 1
+    assert data["status"] == "running"
+    # Background task executed by TestClient: run_sync was called once
+    mock_sync.assert_awaited_once()
 
 
 @pytest.mark.asyncio