feat: Add per-case dir oneliner locks

Introduces `OnelinerLocks` to serialize read-modify-write operations on
`oneliner.json`. This prevents race conditions between the manual
override API (`PUT /api/cases/{case_id}/oneliner`) and the transcription
worker's auto-regeneration process.

The manual override now acquires a lock specific to the `case_dir`
before writing. The transcription worker's `update_oneliner` function
also acquires the same lock around its post-LLM re-read-and-write
sequence. This ensures that a doctor's manual edit always takes
precedence, even if it arrives while an auto-regeneration is in
progress.

This change also includes:
- A new `routes::oneliner_override` module for the PUT handler.
- Validation for the oneliner text length and emptiness.
- Integration tests for the override functionality, covering happy path,
  error conditions, early latching, race conditions, and parallel PUTs.

server/src/paths.rs

@@ -87,10 +87,7 @@ pub async fn read_oneliner_state(
 /// `write(tmp) + rename(tmp, final)`. Rename is atomic within a single
 /// filesystem, so concurrent readers never see a half-serialized JSON
 /// document.
-pub async fn write_oneliner_state(
-    case_dir: &Path,
-    state: &OnelinerState,
-) -> std::io::Result<()> {
+pub async fn write_oneliner_state(case_dir: &Path, state: &OnelinerState) -> std::io::Result<()> {
     let final_path = case_dir.join(ONELINER_FILENAME);
     let tmp_path = case_dir.join(format!("{ONELINER_FILENAME}.tmp"));
     let payload = serde_json::to_vec(state).expect("OnelinerState is infallibly serializable");