refactor: consume agenternal-pkg instead of local memory modules

[hdviettt]

Phase 5: Adapt Agenternal to use the shared package

Replace local backend/memory/, backend/db/models.py memory models, and backend/tools/ with imports from the agenternal pip package.

Steps

1. pip install -e ../agenternal-pkg in backend venv
2. Delete backend/memory/ directory
3. Split backend/db/models.py — keep User, Notification, UserSettings only, import Base from package
4. Update backend/main.py:
   • Add agenternal.configure(...) call in lifespan
   • Wire on_notification and get_memory_config callbacks
5. Update all api/*.py to import from agenternal.*
6. Update agent/graph.py tool imports
7. Update requirements.txt — add agenternal dependency, remove packages now owned by the package
8. Test everything

Risk

• Circular imports from lazy imports — verify at runtime
• Notification callback wiring must match existing signature

Depends on: hdviettt/agenternal-pkg#4