Feat: Add web interface for case listing and audio playback

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.

server/src/error.rs

@@ -5,6 +5,7 @@ use serde_json::json;
 pub enum AppError {
     Unauthorized,
     BadRequest(String),
+    NotFound(String),
     Internal(String),
 }
 
@@ -13,6 +14,7 @@ impl IntoResponse for AppError {
         let (status, message) = match &self {
             Self::Unauthorized => (StatusCode::UNAUTHORIZED, "Unauthorized".to_owned()),
             Self::BadRequest(msg) => (StatusCode::BAD_REQUEST, msg.clone()),
+            Self::NotFound(msg) => (StatusCode::NOT_FOUND, msg.clone()),
             Self::Internal(msg) => (StatusCode::INTERNAL_SERVER_ERROR, msg.clone()),
         };