fix(transcript): collapse rolling segment echoes

Normalize incremental backend hypothesis chains before persistence and ignore stale or replayed webhook callbacks so duplicate transcript text does not survive ingest.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/routes/api/webhook/[jobId]/+server.ts

@@ -12,12 +12,22 @@ const jobId = params.jobId;
 const job = getJob(jobId);
 if (!job) throw error(404, 'Job not found');
 
+const whisperJob = (await request.json()) as WhisperJob;
+
 // Discard the result if the job was cancelled locally while whisper was running
 if (job.status === 'cancelled') {
 return json({ ok: true });
 }
 
-const whisperJob = (await request.json()) as WhisperJob;
+// Ignore stale callbacks from a previous whisper job after a local retry/reset.
+if (job.whisperJobId && whisperJob.id !== job.whisperJobId) {
+return json({ ok: true, ignored: 'stale_whisper_job' });
+}
+
+// Ignore replayed success callbacks after the transcript is already persisted.
+if (job.status === 'done' && job.segmentsJson) {
+return json({ ok: true, ignored: 'duplicate_webhook' });
+}
 
 if (whisperJob.status === 'failed' || whisperJob.status === 'cancelled') {
 const msg = whisperJob.error ?? `Whisper job ${whisperJob.status}`;