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-frontend/src/App.tsx

@@ -20,6 +20,7 @@ const SettingsPage = lazy(() => import('./pages/SettingsPage'));
 const UserSettingsPage = lazy(() => import('./pages/UserSettingsPage'));
 const LoginPage = lazy(() => import('./pages/LoginPage'));
 const DashboardPage = lazy(() => import('./pages/DashboardPage'));
+const FreigabePage = lazy(() => import('./pages/FreigabePage'));
 import { Permission } from './auth/permissions';
 
 function UnauthorizedPage() {
@@ -131,6 +132,7 @@ function ThemedApp() {
                   <Route path="/mailpostfach" element={<PermissionRoute permission={Permission.VIEW_MAIL}><MailpostfachPage /></PermissionRoute>} />
                   <Route path="/mailpostfach/:id" element={<PermissionRoute permission={Permission.VIEW_MAIL}><MailDetailPage /></PermissionRoute>} />
                   <Route path="/settings" element={<PermissionRoute permission={Permission.MANAGE_SETTINGS}><SettingsPage /></PermissionRoute>} />
+                  <Route path="/freigabe" element={<PermissionRoute permission={Permission.VIEW_FREIGABE}><FreigabePage /></PermissionRoute>} />
                   <Route path="/user-settings" element={<UserSettingsPage />} />
                 </Route>
               </Routes>