Configure ruff and mypy

[marcgibbons]

Ruff

Temporary suppresses are in place in pyproject.toml. Work through these incrementally:

• ANN — add type annotations throughout goodconf/ and tests/
• PTH — replace os.path usage with pathlib in goodconf/__init__.py
• TRY003 / TRY004 — refactor exception messages and types
• EM102 — move exception messages out of f-strings
• TC002 — move type-only imports into TYPE_CHECKING blocks
• TID252 — enforce absolute imports

mypy

ignore_errors = true is set as a starter config. 14 known errors exist:

• Type mismatches in the lazy import pattern (_load_config)
• Missing keys in GoodConfConfigDict TypedDict
• Missing Django stubs (django-stubs)

Tighten progressively: remove ignore_errors = true, fix errors one rule group at a time, add django-stubs to dev deps.