chore(deps): require modern-di-fastapi 4

[lesnik512]

Closes #77. Part of modern-python/modern-di#579.

Summary

Bumps modern-di-fastapi to >=4,<5. The lock moves modern-di 3.5.0 to 4.0.0 and modern-di-fastapi 3.0.2 to 4.0.0; nothing else changes.

Design decisions

#77 expected choose_sa_engine(request: fastapi.Request | None = None) to raise ContextValueNotSetError outside a request under 4.0, and proposed an explicit ContextProvider(..., default=None). It was written before modern-python/modern-di#602, which makes a nullable or defaulted parameter fall back when its context value is unset. test_no_request_uses_primary passes unchanged, so no code change is needed.

Test plan

• just test in Docker against Postgres: 31 passed, 100% coverage, with modern-di 4.0.0 and modern-di-fastapi 4.0.0 in the container
• ruff, ty