feat: add Steuertags concept to separate workflow from content tags

- New steuertag_ids setting to mark tags as workflow-only (not editable)
- DocumentEditModal shows only content tags (non-Steuertags) as editable chips
- Backend preserves Steuertags when saving document tag changes
- ManuellBearbeitenPage renders content tag chips under document title
- New Steuertags settings tab with multi-select and color preview

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

paperless-frontend/src/api/settings.ts

@@ -145,6 +145,11 @@ export const settingsApi = {
   deleteUserClient: (id: number) =>
     api.delete(`/api/settings/user-clients/${id}`).then(r => r.data),
 
+  // Steuertags
+  getSteuertagIds: () => api.get<{ ids: number[] }>('/api/settings/steuertags').then(r => r.data.ids),
+  updateSteuertagIds: (ids: number[]) =>
+    api.put<{ ids: number[] }>('/api/settings/steuertags', { ids }).then(r => r.data.ids),
+
   // Korrespondenten
   getCorrespondents: (page = 1, pageSize = 50, search?: string) =>
     api.get<{ data: any[], total: number }>('/api/settings/correspondents', { params: { page, pageSize, search } }).then(r => r.data),