feat: Add file upload endpoint

This commit introduces a new API endpoint `/api/upload` for handling
audio file uploads. It supports multipart form data, extracts `case_id`,
`recorded_at`, and `audio` fields. The audio data is saved to a
filesystem path determined by the user and case ID, within either the
`open/` or `done/` directories.

New features include:
- Integration with `axum`'s `Multipart` extractor for parsing form data.
- Validation of `case_id` as a UUID.
- Creation of case directories if they do not exist.
- Handling of late uploads to already closed cases by placing files in
  the `done/` directory and removing any `.remove` marker.
- Logging of received recordings.
- Definition of `AckResponse` and `AckStatus` models for API responses.
- Addition of comprehensive unit tests for various upload scenarios,
  including new case creation, invalid inputs, authentication, and late
  uploads.

server/src/routes/mod.rs

@@ -1,9 +1,10 @@
 mod debug;
 mod health;
+mod upload;
 
 use std::sync::Arc;
 
-use axum::routing::get;
+use axum::routing::{get, post};
 use axum::Router;
 
 use crate::config::Config;
@@ -12,4 +13,5 @@ pub fn api_router() -> Router<Arc<Config>> {
     Router::new()
         .route("/api/health", get(health::handle_health))
         .route("/api/debug/whoami", get(debug::handle_whoami))
+        .route("/api/upload", post(upload::handle_upload))
 }