fix(transcription): guard @mention fetch against stale responses

Tag each runSearch with an incrementing requestId; discard responses
whose id no longer matches the latest onSearch. Prevents a slow fetch
from repopulating the dropdown after the user has cleared the search.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/lib/shared/discussion/PersonMentionEditor.svelte.spec.ts

@@ -237,6 +237,36 @@ describe('PersonMentionEditor — AC-2/3: search input drives fetch', () => {
 	});
 });
 
+// ─── Stale-response race (Sara on PR #629) ───────────────────────────────────
+
+describe('PersonMentionEditor — stale-response race', () => {
+	it('discards a stale response that resolves after the search has been cleared', async () => {
+		let resolveFetch!: (v: { ok: boolean; json: () => Promise<Person[]> }) => void;
+		const pendingResponse = new Promise<{ ok: boolean; json: () => Promise<Person[]> }>((r) => {
+			resolveFetch = r;
+		});
+		const fetchMock = vi.fn().mockReturnValue(pendingResponse);
+		vi.stubGlobal('fetch', fetchMock);
+		renderHost();
+
+		// Open the dropdown and let the debounce fire so a fetch is in flight.
+		await userEvent.type(page.getByRole('textbox'), '@Aug');
+		await vi.waitFor(() => {
+			expect(fetchMock).toHaveBeenCalledWith(expect.stringContaining('/api/persons?q=Aug'));
+		});
+
+		// Clear the search input *before* the fetch resolves.
+		await userEvent.clear(page.getByRole('searchbox'));
+		await expect.element(page.getByRole('searchbox')).toHaveValue('');
+
+		// The stale fetch now resolves with persons. The dropdown must stay empty.
+		resolveFetch({ ok: true, json: () => Promise.resolve([AUGUSTE]) });
+		await new Promise((r) => setTimeout(r, 50));
+
+		await expect.element(page.getByText('Auguste Raddatz')).not.toBeInTheDocument();
+	});
+});
+
 // ─── AC-1: search input prefilled with text typed after @ ───────────────────
 
 describe('PersonMentionEditor — AC-1: search input prefill', () => {