fix(transcription): neutralize legacy items() to dedupe @mention fetch

Tiptap's suggestion items() callback fired a fetch on every keystroke
after `@`, in parallel with the debounced search-input fetch. Its result
was discarded by updateState, so it was pure waste — doubling the load
on /api/persons and confusing the debounce.

Returning [] from items() routes the entire fetch flow through the
search-input -> debounced onSearch path. New test pins @Walter to
exactly one fetch.

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

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

@@ -151,16 +151,13 @@ onMount(() => {
 					// Nora #5618 #3 — separate issue tracks the GET /api/persons
 					// response-shape audit (PersonSummaryDTO leaks `notes`).
 					// ─────────────────────────────────────────────────────────────
-					items: async ({ query }: { query: string }) => {
-						if (!query) return [];
-						try {
-							const res = await fetch(`/api/persons?q=${encodeURIComponent(query)}`);
-							if (!res.ok) return [];
-							return ((await res.json()) as Person[]).slice(0, 5);
-						} catch {
-							return [];
-						}
-					},
+					// Tiptap's suggestion plugin requires an `items()` callback to keep
+					// the dropdown alive, but the actual fetch is owned by `runSearch`
+					// below — routed through the dropdown's search input via the
+					// debounced `onSearch` channel. Returning `[]` here keeps Tiptap
+					// happy without firing a duplicate per-keystroke fetch.
+					// Markus #5616 / Felix / Nora / Sara on PR #629.
+					items: async () => [],
 					// AC-1 fix: insert the typed query as displayName, not person.displayName.
 					command({ editor: ed, range, props }) {
 						const p = props as unknown as { personId: string; displayName: string };