feat: Add recovery scan for pending recordings

This commit introduces a new module `transcribe::recovery` and a
function `scan_and_enqueue`.
This function is called at startup to find audio files that have not yet
been transcribed
and re-enqueues them for processing. This ensures that recordings are
not lost if the
server restarts before transcription is complete.

A new integration test `recovery_enqueues_only_pending_recordings` has
been added to
verify the functionality.

server/src/main.rs

@@ -50,6 +50,15 @@ async fn main() {
         http_client,
     ));
 
+    // Re-enqueue pending recordings left over from a previous run.
+    {
+        let tx = transcribe_tx.clone();
+        let data_path = config.data_path.clone();
+        tokio::spawn(async move {
+            transcribe::recovery::scan_and_enqueue(&data_path, &tx).await;
+        });
+    }
+
     let state = AppState {
         config: config.clone(),
         transcribe_tx,