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/MentionDropdown.svelte

@@ -40,6 +40,14 @@ $effect(() => {
 	}
 });
 
+// Fire onSearch whenever the effective query changes — covers both the
+// editor mirror and direct input edits. This is the only place onSearch
+// fires; when the dropdown is unmounted, the effect is disposed and no
+// further fetches occur.
+$effect(() => {
+	onSearch(searchQuery);
+});
+
 // highlightedIndex must be both writable (keyboard handler mutates it) and
 // reset when `items` changes (so it never points past the end of a new list).
 // A pure $derived is read-only and cannot serve both needs, so $state + $effect
@@ -161,9 +169,8 @@ function selectItem(item: Person) {
 				class="min-h-[44px] w-full bg-transparent font-sans text-sm text-ink placeholder:text-ink-3 focus:outline-none focus-visible:ring-2 focus-visible:ring-brand-navy focus-visible:ring-inset"
 				placeholder={m.person_mention_search_prompt()}
 				bind:value={searchQuery}
-				oninput={(e) => {
+				oninput={() => {
 					userHasEdited = true;
-					onSearch(e.currentTarget.value);
 				}}
 				onmousedown={(e) => e.stopPropagation()}
 			/>