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

@@ -7,8 +7,12 @@ import { m } from '$lib/paraglide/messages.js';
 import type { components } from '$lib/generated/api';
 import type { PersonMention } from '$lib/shared/types';
 import { deserialize, serialize } from '$lib/shared/discussion/mentionSerializer';
+import { debounce } from '$lib/shared/utils/debounce';
 import MentionDropdown from './MentionDropdown.svelte';
 
+const SEARCH_DEBOUNCE_MS = 150;
+const SEARCH_RESULT_LIMIT = 5;
+
 type Person = components['schemas']['Person'];
 
 type Props = {
@@ -33,6 +37,13 @@ let {
 
 let editorEl: HTMLDivElement;
 let editor: Editor | null = null;
+// Hoisted so onDestroy can guarantee the imperatively-mounted dropdown is
+// torn down even if Tiptap's suggestion plugin onExit didn't fire (e.g. when
+// the host component is unmounted while the dropdown is still open).
+let mountedDropdown: object | null = null;
+// Hoisted so onDestroy can cancel any pending fetch — otherwise a trailing
+// debounced search can fire after the editor is gone and pollute later tests.
+let cancelPendingSearch: (() => void) | null = null;
 
 // Single reactive state object shared with MentionDropdown. Mutating these
 // fields propagates to the mounted dropdown via Svelte's $state proxy —
@@ -167,7 +178,6 @@ onMount(() => {
 							.run();
 					},
 					render() {
-						let component: object | null = null;
 						let exports: DropdownExports | null = null;
 
 						// Tiptap's SuggestionProps types `command` against the default
@@ -180,8 +190,33 @@ onMount(() => {
 							clientRect?: (() => DOMRect | null) | null;
 						};
 
+						const runSearch = async (query: string) => {
+							try {
+								const res = await fetch(`/api/persons?q=${encodeURIComponent(query)}`);
+								if (!res.ok) {
+									dropdownState.items = [];
+									return;
+								}
+								dropdownState.items = ((await res.json()) as Person[]).slice(
+									0,
+									SEARCH_RESULT_LIMIT
+								);
+							} catch {
+								dropdownState.items = [];
+							}
+						};
+						const debouncedSearch = debounce(runSearch, SEARCH_DEBOUNCE_MS);
+						cancelPendingSearch = () => debouncedSearch.cancel();
+						const onSearch = (query: string) => {
+							if (query.trim() === '') {
+								debouncedSearch.cancel();
+								dropdownState.items = [];
+								return;
+							}
+							debouncedSearch(query);
+						};
+
 						const updateState = (renderProps: LooseRenderProps) => {
-							dropdownState.items = renderProps.items as Person[];
 							// AC-1: pass typed query as displayName, not person.displayName
 							dropdownState.command = (item: Person) =>
 								renderProps.command({
@@ -208,10 +243,10 @@ onMount(() => {
 										get editorQuery() {
 											return dropdownState.editorQuery;
 										},
-										onSearch: () => {}
+										onSearch
 									}
 								});
-								component = mounted as object;
+								mountedDropdown = mounted as object;
 								exports = mounted as unknown as DropdownExports;
 							},
 							onUpdate(renderProps) {
@@ -223,9 +258,9 @@ onMount(() => {
 								return exports?.onKeyDown(event) ?? false;
 							},
 							onExit() {
-								if (component) {
-									unmount(component);
-									component = null;
+								if (mountedDropdown) {
+									unmount(mountedDropdown);
+									mountedDropdown = null;
 									exports = null;
 								}
 							}
@@ -268,7 +303,15 @@ onMount(() => {
 });
 
 onDestroy(() => {
+	cancelPendingSearch?.();
 	editor?.destroy();
+	// Tiptap suggestion onExit usually unmounts the dropdown, but if the host
+	// component is destroyed while a suggestion is active the dropdown can
+	// outlive the editor — clean it up explicitly.
+	if (mountedDropdown) {
+		unmount(mountedDropdown);
+		mountedDropdown = null;
+	}
 });
 
 // Keep the data-placeholder attribute in sync with actual emptiness so the