What

Quality-tooling batch (independent of the release-please migration in #2):

• Config hardening — log_level validator (rejects unknown levels), SettingsConfigDict instead of raw dict.
• Logging — add a console StreamHandler alongside the file handler.
• Entry point — use the shared settings instance and model_dump() (drop deprecated .dict()).
• Tooling — add mypy + ruff-format pre-commit hooks; mypy, pytest-cov and coverage config in pyproject.toml; bump ruff to ^0.15.
• CI — python-app.yml installs via Poetry with cache, matrix 3.12 + 3.13.
• Tests — cover main() end-to-end and invalid-log-level rejection.

Notes

First of two stacked PRs. Merge this before #2 (release-please migration), which is branched on top of it.