docs: Update documentation for role-based access control

- Update README.md with admin system documentation
- Add admin setup instructions and role permissions
- Update API endpoint documentation with access requirements
- Update Bruno collection to reflect admin-only operations
- Document shared library concept and security model
- Add comprehensive admin setup guide

bruno/user/Add Ebook Folder.bru

@@ -24,20 +24,20 @@ settings {
 docs {
   ## Add Ebook Folder
   
-  Adds a new ebook folder for the authenticated user.
+  Adds a new ebook folder for scanning. **Admin only operation.**
   
   **Method:** POST
   
   **Endpoint:** /api/auth/ebook-folders
   
-  **Authentication:** Required
+  **Authentication:** Required (Admin role required)
   
   **Request Body:**
   - `folder_path` (string, required): Path to the ebook folder
   
   **Response:**
   - `id` (string): Folder ID
-  - `user_id` (string): User ID
+  - `user_id` (string): Admin user ID
   - `folder_path` (string): Folder path
   - `created_at` (string): Creation timestamp
   
@@ -45,5 +45,6 @@ docs {
   - 201: Created
   - 400: Invalid request
   - 401: Unauthorized
+  - 403: Forbidden (non-admin users)
   - 409: Folder already exists
 }