feat: implement Freigabesystem for payment approval workflow

Adds a dedicated approval view for PM_Freigabe users to release documents
for payment by setting Paperless custom field 15 to a predefined value.

- Backend: VIEW_FREIGABE permission mapped to PM_Freigabe OIDC group
- Backend: FreigabeErforderlich flag on DocumentType entity (auto-migrated)
- Backend: FreigabeModule with endpoints to list documents, fetch field
  options dynamically from Paperless, and set the approval custom field
- Frontend: /freigabe route with filter (default: nicht freigegeben),
  paginated table, and modal to select approval value
- Frontend: Settings checkbox to mark document types as requiring approval
- Frontend: Freigabe menu item visible only to PM_Freigabe/PM_Admin users

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

paperless-backend/src/database/entities/document-type.entity.ts

@@ -16,4 +16,7 @@ export class DocumentType {
 
   @Column({ type: 'int', nullable: true })
   TagReady!: number | null;
+
+  @Column({ type: 'tinyint', width: 1, nullable: true, default: null })
+  FreigabeErforderlich!: boolean | null;
 }