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/pages/SettingsPage.tsx

@@ -579,6 +579,12 @@ function DocTypesTab() {
     { title: 'Titel-Template', dataIndex: 'TitelTemplate', key: 'template', ellipsis: true },
     { title: 'Tag (nicht bereit)', dataIndex: 'TagNotReady', key: 'tagNR', render: getTagName },
     { title: 'Tag (bereit)', dataIndex: 'TagReady', key: 'tagR', render: getTagName },
+    {
+      title: 'Freigabe erf.',
+      dataIndex: 'FreigabeErforderlich',
+      key: 'freigabe',
+      render: (v: boolean | null) => v ? <Tag color="blue">Ja</Tag> : '—',
+    },
     {
       title: '',
       key: 'actions',
@@ -624,6 +630,9 @@ function DocTypesTab() {
               {tags.map(t => <Select.Option key={t.id} value={t.id}>{t.name}</Select.Option>)}
             </Select>
           </Form.Item>
+          <Form.Item name="FreigabeErforderlich" valuePropName="checked" label="Freigabe erforderlich">
+            <Checkbox />
+          </Form.Item>
         </Form>
         
         {editing && (