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/templates/cases.html

@@ -15,6 +15,8 @@ body { font-family: sans-serif; max-width: 900px; margin: 2em auto; padding: 0 1
 .recording-head span { font-family: monospace; font-size: 0.9em; min-width: 20em; }
 .transcript { margin: 0.5em 0 0 1em; padding: 0.5em; background: #f6f6f6; border-radius: 4px; white-space: pre-wrap; font-family: serif; }
 .pending { margin: 0.5em 0 0 1em; color: #888; font-style: italic; }
+.failed { margin: 0.5em 0 0 1em; color: #b00; font-weight: bold; }
+.recording.failed-row { background: #fff7f7; border-left: 3px solid #e24a4a; padding-left: 0.5em; }
 .oneliner { font-size: 1em; color: #333; margin: 0 0 0.3em 0; }
 </style>
 </head>
@@ -33,19 +35,23 @@ body { font-family: sans-serif; max-width: 900px; margin: 2em auto; padding: 0 1
 {% endmatch %}
 <small>{{ case.case_id }}</small>
 {% for rec in case.recordings %}
-<div class="recording">
+<div class="recording{% if rec.failed %} failed-row{% endif %}">
 <div class="recording-head">
 <span>{{ rec.filename }}</span>
 <audio controls preload="none">
 <source src="/web/audio/{{ case.user_slug }}/{{ case.case_id }}/{{ rec.filename }}" type="audio/mp4">
 </audio>
 </div>
+{% if rec.failed %}
+<div class="failed">Transcription failed — rename away the .failed suffix to retry.</div>
+{% else %}
 {% match rec.transcript %}
 {% when Some with (t) %}
 <div class="transcript">{{ t }}</div>
 {% when None %}
 <div class="pending">Transcription pending…</div>
 {% endmatch %}
+{% endif %}
 </div>
 {% endfor %}
 </div>