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/lib/server/whisper.ts

@@ -90,13 +90,16 @@ export async function submitJob(
 	const { createReadStream } = await import('fs');
 	const { default: fetch } = await import('node-fetch');
 
-	const form = new FormData();
-	form.append('audio', createReadStream(wavPath));
-	form.append('task', 'transcribe');
-	form.append('webhook_url', webhookUrl);
-	if (language) form.append('language', language);
-
 	for (let attempt = 1; attempt <= maxAttempts; attempt++) {
+		// Recreate form with a fresh readable stream on every attempt.
+		// A consumed ReadStream cannot be rewound, so reusing it across retries
+		// would send an empty body to whisper after the first 503.
+		const form = new FormData();
+		form.append('audio', createReadStream(wavPath));
+		form.append('task', 'transcribe');
+		form.append('webhook_url', webhookUrl);
+		if (language) form.append('language', language);
+
 		const res = await fetch(`${whisperUrl()}/jobs`, {
 			method: 'POST',
 			body: form,