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/app.module.ts

@@ -20,6 +20,7 @@ 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 { ZahlungModule } from './zahlung/zahlung.module';
 import { DailyDigestModule } from './daily-digest/daily-digest.module';
 import * as path from 'path';
 
@@ -52,6 +53,7 @@ import * as path from 'path';
     LabelPrintAgentModule,
     AgrarmonitorModule,
     FreigabeModule,
+    ZahlungModule,
     DailyDigestModule,
   ],
 })