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

@@ -0,0 +1,13 @@
+import { json } from '@sveltejs/kit';
+import { unloadModel } from '$lib/server/whisper.js';
+
+/** Proxy for POST /model/unload on the whisper backend. */
+export async function POST() {
+	try {
+		const body = await unloadModel();
+		return json(body);
+	} catch (err: unknown) {
+		const message = err instanceof Error ? err.message : String(err);
+		return json({ ok: false, error: message }, { status: 502 });
+	}
+}