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/routes/web.rs

@@ -14,6 +14,9 @@ use crate::error::AppError;
 struct RecordingView {
     filename: String,
     transcript: Option<String>,
+    /// True if the file has been renamed to `<ts>.m4a.failed` by the worker
+    /// after a non-recoverable ffmpeg or whisper error.
+    failed: bool,
 }
 
 struct CaseView {
@@ -80,7 +83,8 @@ fn validate_user_slug(user: &str) -> Result<(), AppError> {
 }
 
 fn validate_filename(filename: &str) -> Result<(), AppError> {
-    if !filename.ends_with(".m4a")
+    let ok_suffix = filename.ends_with(".m4a") || filename.ends_with(".m4a.failed");
+    if !ok_suffix
         || filename.contains('/')
         || filename.contains('\\')
         || filename.contains("..")
@@ -178,12 +182,21 @@ async fn scan_recordings(case_dir: &Path) -> Vec<RecordingView> {
             Ok(s) => s,
             Err(_) => continue,
         };
-        if !filename.ends_with(".m4a") {
+        let failed = filename.ends_with(".m4a.failed");
+        let is_audio = filename.ends_with(".m4a") || failed;
+        if !is_audio {
             continue;
         }
-        let transcript_path = case_dir.join(&filename).with_extension("transcript.txt");
+        // Transcript naming uses the original `<ts>.m4a` stem in both cases
+        // (failed recordings never produced one, but the lookup still works).
+        let stem_path = case_dir.join(filename.trim_end_matches(".failed"));
+        let transcript_path = stem_path.with_extension("transcript.txt");
         let transcript = tokio::fs::read_to_string(&transcript_path).await.ok();
-        recordings.push(RecordingView { filename, transcript });
+        recordings.push(RecordingView {
+            filename,
+            transcript,
+            failed,
+        });
     }
 
     recordings.sort_by(|a, b| a.filename.cmp(&b.filename));