Make Mypy more restrictive

[bokelley]

Tighten up Mypy:

Our current config (lenient, incremental adoption):

• ✅ show_error_codes = True - MATCHES Wolt
• ❌ warn_return_any = False - Wolt: True
• ❌ disallow_untyped_defs = False - Wolt: True (most important!)
• ❌ check_untyped_defs = False - Wolt: True
• ❌ no_implicit_optional = False - Wolt: True
• ❌ ignore_missing_imports = True - Wolt recommends disallow_any_unimported = True

Current status: We're at 0 mypy errors with lenient settings. This is excellent progress!

Recommendation: Now that we have 0 errors, we should gradually tighten strictness:

Phase 1 (next PR):

1. Enable no_implicit_optional = True - explicit Optional annotations
2. Enable warn_return_any = True - catch Any leakage
3. Enable check_untyped_defs = True - check untyped function bodies

Phase 2 (future):
4. Enable disallow_untyped_defs = True - require type hints (biggest impact)
5. Replace ignore_missing_imports = True with disallow_any_unimported = True

The key insight from Wolt: start strict for new code, be lenient with legacy. We could enable strict
settings globally and use per-module overrides for existing code.