Introduces a new web interface to list cases and play back audio
recordings. This includes:
- A new `web` module in `routes` to handle web requests.
- `handle_case_list` to scan and display available cases and their
recordings.
- `handle_audio` to serve audio files, with validation to prevent path
traversal and ensure correct file types.
- `AppError::NotFound` variant to handle missing resources gracefully.
- Updates to `projektplan.md` to document the change in STT container
and m4a preprocessing.
- New tests in `server/tests/web_test.rs` to ensure the web interface
functions correctly.
