Add recovery for missing oneliners

This commit introduces a recovery mechanism to regenerate `oneliner.txt`
files for cases where transcription was successful but the oneliner
generation failed due to a crash between writing the transcript and the
oneliner.

The `transcribe::recovery::regenerate_missing_oneliners` function is
added and called at startup after `scan_and_enqueue`. This function
identifies cases with non-empty transcripts but missing oneliners and
triggers their regeneration.

The `transcribe::worker::update_oneliner` function is refactored to
always regenerate the oneliner from all available transcripts in a case,
ensuring that later recordings can correct earlier ones. This replaces
the previous `ensure_oneliner` logic which only generated the oneliner
if it didn't exist.

New helper functions `cases_needing_oneliner` and
`all_transcripts_joined` are introduced to support these changes.
Comprehensive unit tests are included for the new recovery and file
processing logic.

server/src/main.rs

@@ -77,8 +77,13 @@ async fn main() {
     {
         let tx = transcribe_tx.clone();
         let data_path = config.data_path.clone();
+        let client = http_client.clone();
+        let cfg = config.clone();
         tokio::spawn(async move {
             transcribe::recovery::scan_and_enqueue(&data_path, &tx).await;
+            // Then catch up oneliners for cases that crashed between
+            // transcript-write and oneliner-write in a previous run.
+            transcribe::recovery::regenerate_missing_oneliners(&data_path, &client, &cfg).await;
         });
     }