refactor: merge admin/learning into admin-api/learning, restore API isolation

- Move AdminLearningService + DTOs to learning-session module
- Merge 21 new endpoints into existing admin-api/learning controller
- Add analysis and ai-usage methods to unified service
- Delete admin-learning module (no longer needed)
- Revert JwtAuthGuard /api/admin bypass (was breaking isolation)
- Fix: /api/* now exclusively serves user/iOS traffic again

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/services/learningAdmin.ts

@@ -1,4 +1,4 @@
-const BASE = '/api/admin/learning';
+const BASE = '/admin-api/learning';
 
 export interface PaginatedResponse<T> {
   items: T[];