feat(i18n): inject user language into AI agent system prompts

- Add _language_instruction() to deep_agent.py, reads language from core memory
- Append language directive to all 4 run_* functions (task/project/checkpoint/note)
- Minor fixes: alembic env, route imports, test cleanup

tests/test_device_ws.py

@@ -18,13 +18,12 @@ from datetime import datetime, timezone
 from unittest.mock import AsyncMock, MagicMock, patch
 
 import pytest
-import pytest_asyncio
 
-from app.core.device_manager import DeviceConnection, DeviceConnectionManager
+from app.core.device_manager import DeviceConnectionManager
 from app.db import get_session
 from app.main import app
 from app.models import AgentRunLog
-from tests.conftest import TEST_USER_IDS, auth_header, make_jwt
+from tests.conftest import TEST_USER_IDS, make_jwt
 
 # ---------------------------------------------------------------------------
 # Helpers