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/routes/api/model/events/+server.ts

@@ -0,0 +1,43 @@
+const WHISPER_URL = process.env.WHISPER_URL ?? 'http://localhost:8080';
+
+/** Relay the whisper /model/events SSE stream to the browser. */
+export async function GET({ request }) {
+	const { default: fetch } = await import('node-fetch');
+
+	const ac = new AbortController();
+	request.signal.addEventListener('abort', () => ac.abort());
+
+	const stream = new ReadableStream({
+		async start(controller) {
+			try {
+				const upstream = await fetch(`${WHISPER_URL}/model/events`, {
+					signal: ac.signal as AbortSignal
+				});
+				if (!upstream.body) {
+					controller.close();
+					return;
+				}
+				for await (const chunk of upstream.body) {
+					if (ac.signal.aborted) break;
+					controller.enqueue(chunk instanceof Buffer ? chunk : Buffer.from(String(chunk)));
+				}
+			} catch {
+				// upstream closed, client disconnected, or whisper unreachable — all fine
+			} finally {
+				controller.close();
+			}
+		},
+		cancel() {
+			ac.abort();
+		}
+	});
+
+	return new Response(stream, {
+		headers: {
+			'Content-Type': 'text/event-stream',
+			'Cache-Control': 'no-cache',
+			Connection: 'keep-alive',
+			'X-Accel-Buffering': 'no'
+		}
+	});
+}