feat: provide settings to the database engine through the container

[lesnik512]

Brings chat-app in line with modern-python/litestar-sqlalchemy-template#81 and modern-python/fastapi-sqlalchemy-template#104: the database engine gets its settings from the container instead of the module-level settings.

Changes

• ioc.Config.settings is a cached Factory(Settings).
• create_database_engine(settings: Settings) is autowired by type, the same way database_session already gets its engine.
• New test: a fresh container with Config.settings overridden builds an engine with the overridden pool size.
• Tests that build an engine by hand pass the module-level settings explicitly.
• AGENTS.md lists the Config group and says which code still reads the global.

Design notes

• There is one engine, so there is no derive here. It only helps when components need different values from the same settings. No modern-di bump is needed.
• app/api/auth.py builds jwt_cookie_auth at import time, before any container exists, so it keeps reading the global, as do __main__, migrations/env.py and build_app.

Testing

just test: 155 passed, 100% coverage. ruff and ty are clean.