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/app.module.ts

@@ -19,6 +19,7 @@ import { InboxPostprocessorModule } from './inbox-postprocessor/inbox-postproces
 import { UserSettingsModule } from './user-settings/user-settings.module';
 import { LabelPrintAgentModule } from './label-print-agent/label-print-agent.module';
 import { AgrarmonitorModule } from './agrarmonitor/agrarmonitor.module';
+import { FreigabeModule } from './freigabe/freigabe.module';
 import * as path from 'path';
 
 @Module({
@@ -49,6 +50,7 @@ import * as path from 'path';
     UserSettingsModule,
     LabelPrintAgentModule,
     AgrarmonitorModule,
+    FreigabeModule,
   ],
 })
 export class AppModule {}