feat: implement two-step Freigabe→Zahlung workflow

Adds a payment step after document approval: PM_Freigabe approves
(Field 15 = "freigegeben"), then PM_Zahlung can mark as paid (Field 16).

- Backend: VIEW_ZAHLUNG permission mapped to PM_Zahlung OIDC group
- Backend: ZahlungModule with endpoints to list documents by filter
  (ausstehend/freigegeben/alle), set Field 16, fetch options from Paperless
- Backend: setZahlung() throws ForbiddenException if Field 15 ≠ "freigegeben"
- Frontend: /zahlung route with 3-way filter, two status columns (Freigabe + Zahlung)
- Frontend: "Zahlung verbuchen" button disabled with tooltip for non-approved docs
- Frontend: Zahlung menu item with EuroOutlined icon

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

paperless-backend/src/zahlung/zahlung.module.ts

@@ -0,0 +1,13 @@
+import { Module } from '@nestjs/common';
+import { TypeOrmModule } from '@nestjs/typeorm';
+import { DocumentType } from '../database/entities/document-type.entity';
+import { PaperlessModule } from '../paperless/paperless.module';
+import { ZahlungController } from './zahlung.controller';
+import { ZahlungService } from './zahlung.service';
+
+@Module({
+  imports: [TypeOrmModule.forFeature([DocumentType]), PaperlessModule],
+  controllers: [ZahlungController],
+  providers: [ZahlungService],
+})
+export class ZahlungModule {}