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/Cargo.lock

@@ -92,6 +92,7 @@ dependencies = [
  "matchit",
  "memchr",
  "mime",
+ "multer",
  "percent-encoding",
  "pin-project-lite",
  "serde_core",
@@ -933,6 +934,23 @@ dependencies = [
  "windows-sys 0.61.2",
 ]
 
+[[package]]
+name = "multer"
+version = "3.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "83e87776546dc87511aa5ee218730c92b666d7264ab6ed41f9d215af9cd5224b"
+dependencies = [
+ "bytes",
+ "encoding_rs",
+ "futures-util",
+ "http",
+ "httparse",
+ "memchr",
+ "mime",
+ "spin",
+ "version_check",
+]
+
 [[package]]
 name = "native-tls"
 version = "0.2.18"
@@ -1469,6 +1487,12 @@ dependencies = [
  "windows-sys 0.61.2",
 ]
 
+[[package]]
+name = "spin"
+version = "0.9.8"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6980e8d7511241f8acf4aebddbb1ff938df5eebe98691418c4468d0b72a96a67"
+
 [[package]]
 name = "stable_deref_trait"
 version = "1.2.1"