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_middleware.py

@@ -19,7 +19,7 @@ import pytest_asyncio
 from cryptography.fernet import Fernet
 from sqlalchemy import select
 
-from app.core.memory_middleware import MemoryMiddleware, _PROACTIVE_CONFIDENCE_THRESHOLD
+from app.core.memory_middleware import MemoryMiddleware
 from app.db import get_session
 from app.main import app
 from app.models import (