refactor(#240): remove needsExpert feature completely

Drops the needsExpert / needs_expert flag end-to-end: DB migration
(V37, never applied), Document entity field, PATCH endpoint, service
method, DTO field, all three queue queries, ExpertBadge component,
i18n key, generated API types, and test fixture.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/components/MissionControlStrip.svelte

@@ -8,7 +8,6 @@ type TranscriptionQueueItemDTO = {
 	id: string;
 	title: string;
 	documentDate?: string;
-	needsExpert: boolean;
 	annotationCount: number;
 	textedBlockCount: number;
 	reviewedBlockCount: number;