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.

client-desktop/src/app.rs

@@ -521,9 +521,9 @@ impl DoctateApp {
 
                 let time_str = extract_hhmm(&marker.last_activity_at);
                 let oneliner = match &marker.oneliner {
-                    Some(OnelinerState::Ready { text, .. } | OnelinerState::Manual { text, .. }) => {
-                        text.clone()
-                    }
+                    Some(
+                        OnelinerState::Ready { text, .. } | OnelinerState::Manual { text, .. },
+                    ) => text.clone(),
                     Some(OnelinerState::Empty { .. }) => "∅".to_owned(),
                     Some(OnelinerState::Error { .. }) => "⚠".to_owned(),
                     None => "⏳".to_owned(),