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_memory_models.py

@@ -7,10 +7,9 @@ column is stored as JSON in tests (SQLite-compatible).
 from __future__ import annotations
 
 import uuid
-from datetime import datetime, timezone
+from datetime import datetime
 
 import pytest
-import pytest_asyncio
 from cryptography.fernet import Fernet
 from sqlalchemy import select