feat(transcription): drive @mention fetch through the dropdown search input

For issue #380 (AC-2, AC-3, AC-4 + NFR debounce).

The search input is now the single fetch trigger. The dropdown's
searchQuery reactivity calls onSearch on every change — whether sourced
from the editor mirror or the user's own input. PersonMentionEditor
debounces these calls at 150 ms, short-circuits on empty queries (no
fetch, items cleared), and tears down pending timers on destroy.

The Tiptap suggestion plugin's items() now returns [] — per-keystroke
fetches in the editor are gone. The same /api/persons?q= endpoint is
used; the difference is in when and how often the request fires.

Adds a cancel() method to the debounce utility so destroyed editors
don't leave trailing fetches alive (which previously polluted the test
ledger and would have wasted bandwidth in production tab-close races).

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

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

@@ -161,6 +161,64 @@ describe('PersonMentionEditor — typeahead', () => {
 	});
 });
 
+// ─── AC-2/3: search input drives the person fetch (debounced) ───────────────
+
+describe('PersonMentionEditor — AC-2/3: search input drives fetch', () => {
+	it('editing the search input fires a debounced fetch with the new query', async () => {
+		const fetchMock = vi
+			.fn()
+			.mockResolvedValue({ ok: true, json: vi.fn().mockResolvedValue([AUGUSTE]) });
+		vi.stubGlobal('fetch', fetchMock);
+		renderHost();
+
+		// Open the dropdown so the search input is reachable.
+		await userEvent.type(page.getByRole('textbox'), '@');
+		await vi.waitFor(async () => {
+			await expect.element(page.getByRole('searchbox')).toBeVisible();
+		});
+
+		const fetchesBeforeSearch = fetchMock.mock.calls.length;
+
+		// `fill` simulates a single input event with the final value — sidesteps
+		// per-keystroke timing of userEvent.type so the test can deterministically
+		// assert that one input event collapses into one debounced fetch.
+		await page.getByRole('searchbox').fill('Walter');
+
+		await vi.waitFor(
+			() => {
+				expect(fetchMock).toHaveBeenCalledWith(expect.stringContaining('q=Walter'));
+			},
+			{ timeout: 1000 }
+		);
+
+		const fetchesAfterSearch = fetchMock.mock.calls.length - fetchesBeforeSearch;
+		expect(fetchesAfterSearch).toBe(1);
+	});
+
+	it('clearing the search input clears the list without firing a fetch', async () => {
+		const fetchMock = vi
+			.fn()
+			.mockResolvedValue({ ok: true, json: vi.fn().mockResolvedValue([AUGUSTE]) });
+		vi.stubGlobal('fetch', fetchMock);
+		renderHost();
+
+		await userEvent.type(page.getByRole('textbox'), '@Aug');
+		await vi.waitFor(async () => {
+			await expect.element(page.getByText('Auguste Raddatz')).toBeInTheDocument();
+		});
+
+		const fetchesBeforeClear = fetchMock.mock.calls.length;
+
+		await userEvent.clear(page.getByRole('searchbox'));
+
+		// Wait beyond the debounce window to confirm no fetch was scheduled.
+		await new Promise((r) => setTimeout(r, 250));
+
+		expect(fetchMock.mock.calls.length).toBe(fetchesBeforeClear);
+		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', () => {