feat(update-check): add robust update check with interval support, state management, and CLI integration
- Implement version and interval-based update checks with configurable settings
- Add CLI command `kleinanzeigen-bot update-check` for manual checks
- Introduce state file with versioning, UTC timestamps, and migration logic
- Validate and normalize intervals (1d–4w) with fallback for invalid values
- Ensure correct handling of timezones and elapsed checks
- Improve error handling, logging, and internationalization (i18n)
- Add comprehensive test coverage for config, interval logic, migration, and CLI
- Align default config, translations, and schema with new functionality
- Improve help command UX by avoiding config/log loading for `--help`
- Update documentation and README with full feature overview
- Remove unused translation entries from translations.de.yaml
- Improve translation test to better detect obsolete entries
- Add KNOWN_NEEDED_MODULES for special cases
- Add helper function _message_exists_in_code for better translation verification
- Improve error messages to show both original and translated text
- Fix import sorting in test file
This commit improves the maintainability of the translation system by
removing unused entries and enhancing the verification process.
This commit implements a new 'changed' selector for the --ads option that
publishes only ads that have been modified since their last publication.
The 'due' selector now only republishes ads based on the time interval,
without considering content changes.
The implementation allows combining selectors with commas (e.g., --ads=changed,due)
to publish both changed and due ads. Documentation and translations have been
updated accordingly.
Fixes#411
