feat: Mark failed recordings with .m4a.failed

Add a new `failed` field to `RecordingView` and corresponding logic in
the worker.
When FFmpeg remuxing or Whisper transcription fails, the worker now
renames the
original `.m4a` file to `.m4a.failed`. This prevents the recovery scan
from
reprocessing these files and signals to the UI that transcription
failed.

The UI displays a message indicating failure and suggests renaming the
file
back to `.m4a` to retry. A new integration test
`worker_renames_audio_to_failed_on_whisper_error`
is included to verify this behavior.

server/src/transcribe/worker.rs

@@ -39,6 +39,7 @@ pub async fn run(mut rx: TranscribeReceiver, config: Arc<Config>, client: reqwes
             Ok(f) => f,
             Err(e) => {
                 error!(audio = %audio_path.display(), error = %e, "ffmpeg remux failed");
+                mark_failed(&audio_path).await;
                 continue;
             }
         };
@@ -47,6 +48,7 @@ pub async fn run(mut rx: TranscribeReceiver, config: Arc<Config>, client: reqwes
             Ok(t) => t,
             Err(e) => {
                 error!(audio = %audio_path.display(), error = %e, "whisper call failed");
+                mark_failed(&audio_path).await;
                 continue;
             }
         };
@@ -70,6 +72,29 @@ pub async fn run(mut rx: TranscribeReceiver, config: Arc<Config>, client: reqwes
     warn!("Transcription worker stopped (channel closed)");
 }
 
+/// Atomically rename a failed recording from `<ts>.m4a` to `<ts>.m4a.failed`
+/// so the recovery scan no longer picks it up. The UI still surfaces these
+/// files (with a "failed" flag) so the user can listen to the audio and
+/// manually decide whether to retry (by renaming back) or to delete.
+async fn mark_failed(audio_path: &Path) {
+    let failed_path = audio_path.with_extension("m4a.failed");
+    if let Err(e) = tokio::fs::rename(audio_path, &failed_path).await {
+        // Nothing to roll back — if rename fails, the worst case is that we
+        // retry the same file on next restart. Log and move on.
+        error!(
+            audio = %audio_path.display(),
+            error = %e,
+            "Failed to mark recording as failed; may be retried on restart",
+        );
+    } else {
+        warn!(
+            audio = %audio_path.display(),
+            failed = %failed_path.display(),
+            "Recording marked as failed",
+        );
+    }
+}
+
 /// Generate `case_dir/oneliner.txt` from the given transcript iff the file
 /// does not already exist. Errors are non-fatal: logged and swallowed.
 async fn ensure_oneliner(