Modernized codebase with tooling configuration and CI/CD workflows.

- Replaced legacy typing (Optional, List, Type, Union, Tuple) with PEP 604/585 equivalents.
- Added pyproject.toml with configurations for hatch-vcs, mypy, ruff, and codespell.
- Added CI workflows for formatting, linting, type checking, and spelling.
- Added CD workflow for building and uploading plugin artifacts on push to master and version tags.
- Added dependency auditing workflow with pip-audit.
- Added comprehensive docstrings and inline comments across all modules.
- Fixed User-Agent header using hardcoded version instead of actual plugin version.
- Fixed grammar and terminology in log messages and comments.
- Removed unreachable error handling branch in unsubscribe command.

.gitignore

@@ -1,3 +1,5 @@
 __pycache__/
 *.py[cod]
 *$py.class
+.venv/
+owncastsentry/_version.py