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/ebooks/Create Ebook.bru

@@ -46,9 +46,9 @@ settings {
 docs {
   ## Create Ebook
   
-  Creates a new ebook entry in the library.
+  Creates a new ebook entry in the library. **Admin only operation.**
   
-  **Authentication:** Required (Bearer token)
+  **Authentication:** Required (Bearer token + Admin role)
   
    **Request Body:**
    - `title` (string, required): Book title
@@ -67,9 +67,12 @@ docs {
    - `publisher` (string, optional): Publisher name
    - `contributors` (string, optional): Additional contributors
   
-  **Response:** Created ebook object
+  **Response:** Created ebook object (with added_by_admin_id field)
   
   **Error Responses:**
   - 401: Invalid authentication
+  - 403: Forbidden (non-admin users)
   - 400: Invalid request data
+  
+  **Note:** Ebook will be automatically associated with the admin user who creates it.
 }