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_agent_runner.py

@@ -28,7 +28,6 @@ from datetime import datetime, timezone
 from unittest.mock import AsyncMock, MagicMock, patch
 
 import pytest
-import pytest_asyncio
 
 from app.core.agent_runner import (
     _extract_items_from_content,
@@ -597,7 +596,7 @@ async def test_run_cloud_agent_provider_fetch_error():
 @pytest.mark.asyncio
 async def test_run_cloud_agent_refreshed_token_persisted():
     """When the provider refreshes its token, the new ciphertext is written to DB."""
-    from app.integrations import EmailMessage, encrypt_token
+    from app.integrations import encrypt_token
     from cryptography.fernet import Fernet as _Fernet
 
     fernet_key = _Fernet.generate_key().decode()