docs: add error handling patterns and module dependencies to CLAUDE.md

- Document 3 graceful degradation patterns with code examples
- Add Service Fallback Matrix for optional services
- Add module dependency tree with change impact guide

chore: fix typecheck environment

- Add mypy to dev dependencies in pyproject.toml
- Fix tox.ini typecheck environment to install mypy explicitly

tox.ini

@@ -38,6 +38,9 @@ commands =
 
 [testenv:typecheck]
 description = Static type checking with mypy
+commands_pre =
+deps =
+    mypy>=1.0.0
 commands =
     mypy src --ignore-missing-imports --no-error-summary