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/models.rs

@@ -0,0 +1,15 @@
+use serde::Serialize;
+
+#[derive(Serialize)]
+pub struct AckResponse {
+    pub case_id: String,
+    pub recorded_at: String,
+    pub status: AckStatus,
+}
+
+#[derive(Serialize)]
+#[serde(rename_all = "lowercase")]
+pub enum AckStatus {
+    Received,
+    Gone,
+}