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/messages/de.json

@@ -573,7 +573,6 @@
 	"mission_control_ready_empty": "Noch keine Dokumente vollständig transkribiert.",
 	"mission_control_ready_empty_cta": "Jetzt mitmachen",
 	"mission_control_weekly_pulse": "↑ +{count} diese Woche",
-	"mission_control_expert_badge": "Experten gesucht",
 	"mission_control_blocks_progress": "{texted} / {total} Blöcke",
 	"mission_control_reviewed_pct": "{pct}% geprüft"
 }

frontend/messages/en.json

@@ -573,7 +573,6 @@
 	"mission_control_ready_empty": "No documents fully transcribed yet.",
 	"mission_control_ready_empty_cta": "Start contributing",
 	"mission_control_weekly_pulse": "↑ +{count} this week",
-	"mission_control_expert_badge": "Expert needed",
 	"mission_control_blocks_progress": "{texted} / {total} blocks",
 	"mission_control_reviewed_pct": "{pct}% reviewed"
 }

frontend/messages/es.json

@@ -573,7 +573,6 @@
 	"mission_control_ready_empty": "Aún no hay documentos completamente transcritos.",
 	"mission_control_ready_empty_cta": "Empezar a colaborar",
 	"mission_control_weekly_pulse": "↑ +{count} esta semana",
-	"mission_control_expert_badge": "Se busca experto",
 	"mission_control_blocks_progress": "{texted} / {total} bloques",
 	"mission_control_reviewed_pct": "{pct}% revisado"
 }