feat: Implement CRUD management for absence codes

[PPeitsch]

Description

This pull request introduces a full CRUD (Create, Read, Update, Delete) management system for absence codes, making the application significantly more flexible and adaptable to real-world use cases. The static, hardcoded list of absence types has been replaced with a dynamic, user-configurable system.

Key components of this feature include:

1. New Settings Page: A new management interface has been created at /settings/absences. Users can now add, edit, and delete their own custom absence codes through an interactive, asynchronous UI.
2. Backend API: A new settings blueprint provides a set of RESTful API endpoints to handle all CRUD operations for AbsenceCode records. The API includes robust validation and error handling, such as preventing the creation of duplicate codes and the deletion of codes that are currently in use.
3. Dynamic Integration:
   • The dropdown menu in the "Manual Entry" form is now dynamically populated from the database, ensuring it always reflects the user's custom list of codes.
   • The "Calendar Log" view has been updated to consume the new centralized API endpoint for absence codes.
4. Comprehensive Testing: A new test suite, tests/test_settings.py, has been added, providing 100% code coverage for all new backend logic and ensuring the reliability of the CRUD operations.

This feature is a major step towards making TimeTrack a production-ready tool.

Type of Change

• New feature
• Refactoring
• Test coverage

Testing

• Added a new test file, tests/test_settings.py, with complete coverage for the new settings blueprint.
• Tested all API endpoints for successful operations (GET, POST, PUT, DELETE) and error handling (conflicts, not found, invalid payloads, DB errors).
• Manually verified the entire user flow:
  • Navigated to the new "Settings" page.
  • Successfully created, renamed, and deleted absence codes.
  • Verified that changes made in Settings are immediately reflected in the dropdowns of both the "Manual Entry" form and the "Calendar Log" modal.
  • Confirmed that attempting to delete a code currently in use displays a user-friendly error message.
• Ran the full existing test suite to ensure no regressions were introduced.

Checklist

• I have followed the project's code style (Black, isort, PEP 8)
• My code generates no new warnings
• I have added tests for new functionality
• All tests pass locally
• I have updated the documentation where necessary (Changelog updated)
• I have run pre-commit hooks before submitting

Fixes #6

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
app/__init__.py	100.00% <100.00%> (ø)
app/routes/manual_entry.py	72.41% <100.00%> (ø)
app/routes/monthly_log.py	100.00% <ø> (ø)
app/routes/settings.py	100.00% <100.00%> (ø)

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