feat: model-on-demand lifecycle — retry on 503, live status pill, warming indicator

- whisper.ts: add getModelStatus(); fix submitJob() to retry on 503 using
  Retry-After header instead of throwing; optional onModelWaiting callback
  lets the pipeline surface model state to the UI during the wait
- pipeline.ts: pass onModelWaiting callback → emits model_warming SSE event
  so the job detail page can show 'Warming up model…' while waiting
- types.ts: add ModelStateTag union and ModelStatus interface
- api/model/status: GET route proxies whisper /model/status (falls back to
  {state:'unloaded'} if whisper unreachable)
- api/model/events: GET route relays whisper SSE stream to the browser;
  AbortController tied to request.signal cleans up on disconnect
- layout.svelte: status pill is now live — initial fetch + EventSource on
  /api/model/events; dot colour + label reflect real model state with a
  pulsing animation while loading or waiting_for_gpu
- jobs/[id]/+page.svelte: handle model_warming event type → show a yellow
  'Warming up model…' sub-label with spinner inside the progress card
- whisper.test.ts: update submitJob mocks to status:202 to match real API

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

src/lib/server/pipeline.ts

@@ -129,7 +129,9 @@ async function runJob(
 		emitProgress(jobId, { type: 'status', status: 'transcribing' });
 
 		const webhookUrl = `${WEBHOOK_BASE_URL}/api/webhook/${jobId}`;
-		const whisperJobId = await submitJob(wavPath, webhookUrl, language);
+		const whisperJobId = await submitJob(wavPath, webhookUrl, language, (state, retryAfterSecs) => {
+			emitProgress(jobId, { type: 'model_warming', state, retryAfterSecs });
+		});
 		updateJob({ id: jobId, whisperJobId });
 
 		// ── 5. Open SSE for live progress (non-blocking relay) ───────────────