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/routes/briefwechsel/page.svelte.spec.ts

@@ -39,7 +39,6 @@ const makeDoc = (overrides: Record<string, unknown> = {}) => ({
 	location: 'Berlin',
 	metadataComplete: false,
 	scriptType: 'UNKNOWN' as const,
-	needsExpert: false,
 	sender: { id: 'p1', firstName: 'Hans', lastName: 'Müller' },
 	receivers: [{ id: 'p2', firstName: 'Anna', lastName: 'Schmidt' }],
 	tags: [],