typing: expand return annotations beyond core modules

[Lyellr88]

Track the remaining work to expand type coverage beyond the current core-only scope.

Current state:

• pyproject.toml has disallow_untyped_defs = true.
• Mypy coverage is intentionally scoped to the core memory/model/limiter code where the type relationships matter most.
• Endpoint and middleware functions still return plain dicts in many places, so turning mypy loose across those folders would create a lot of low-signal annotation churn.

Work to do:

• Decide which endpoint and middleware modules are stable enough to annotate first.
• Add return annotations in small batches.
• Prefer typed response models where FastAPI response shape is stable.
• Expand the mypy command once the added annotations produce useful signal.

Why this matters:
This is not urgent, but it is a good cleanup pass once the release churn settles. It should make endpoint refactors safer without forcing fake precision into unstable code.