fix: FormData stream exhausted on retry + undefined segments crash

Two bugs triggered together when the model was unloaded during a job:

1. submitJob() created FormData/createReadStream once outside the retry loop.
   After a 503, the audio ReadStream was consumed and subsequent retries sent
   an empty body to whisper, causing it to return segments:undefined.

2. webhook handler cast whisperJob.segments as Segment[] without guarding
   against undefined, so deduplicateSegments(undefined) crashed with
   'Cannot read properties of undefined (reading 'map')' — stored as job.error.

Fixes:
- Move FormData + createReadStream inside the retry loop (fresh stream per attempt)
- Use (whisperJob.segments ?? []) in webhook handler
- Add Array.isArray guard at top of deduplicateSegments() as belt-and-suspenders

Tests:
- New: verifies createReadStream called once per attempt (3 attempts = 3 streams)
- New: webhook handles segments:undefined without throwing
- New: webhook handles segments:null without throwing
- 150/150 passing

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

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

@@ -30,7 +30,7 @@ try {
 setJobStatus(jobId, 'processing', 90);
 emitProgress(jobId, { type: 'status', status: 'processing', progress: 90 });
 
-const rawSegments = whisperJob.segments as Segment[];
+const rawSegments = (whisperJob.segments ?? []) as Segment[];
 const segments = deduplicateSegments(rawSegments);
 
 const paths = await writeOutputs(segments, job.title, jobId);