feat: proxy POST /model/unload endpoint

- Add unloadModel() to whisper.ts: POSTs to /model/unload with 10s
  timeout, returns parsed JSON body, throws on non-ok response
- Create src/routes/api/model/unload/+server.ts: thin POST proxy,
  passes whisper's response through, returns 502 if whisper unreachable
- Add 3 unloadModel tests (success, WHISPER_URL config, error propagation)
  — 147/147 passing

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

src/lib/server/whisper.ts

@@ -127,6 +127,17 @@ export async function submitJob(
 	throw new Error(`Whisper model did not become ready after ${maxAttempts} attempts`);
 }
 
+/** Unload the model from VRAM. Throws if the whisper server returns non-ok. */
+export async function unloadModel(): Promise<{ ok: boolean }> {
+	const { default: fetch } = await import('node-fetch');
+	const res = await fetch(`${whisperUrl()}/model/unload`, {
+		method: 'POST',
+		signal: AbortSignal.timeout(10000)
+	});
+	if (!res.ok) throw new Error(`/model/unload returned ${res.status}`);
+	return res.json() as Promise<{ ok: boolean }>;
+}
+
 /**
  * Cancel a queued or running job on the whisper server (best-effort).
  * Errors are silently ignored — local job status is already set to cancelled.