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/scanner/Scan Ebooks.bru

@@ -18,16 +18,16 @@ settings {
 docs {
   ## Scan Ebooks
   
-  Scans the user's configured ebook folders for ebooks and adds them to the database.
+  Scans configured ebook folders for ebooks and adds them to the database. **Admin only operation.**
   
   **Method:** POST
   
   **Endpoint:** /api/scanner/scan
   
-  **Authentication:** Required
+  **Authentication:** Required (Admin role required)
   
   **Request Body:**
-  - `folder_paths` (array of strings, optional): Array of folder paths to scan. If not provided, uses user's saved folders from Add Ebook Folder.
+  - `folder_paths` (array of strings, optional): Array of folder paths to scan. If not provided, uses configured folders from Add Ebook Folder.
   
   **Response:**
   - `message` (string): Success message
@@ -36,8 +36,11 @@ docs {
   - 200: Success
   - 400: Bad Request
   - 401: Unauthorized
+  - 403: Forbidden (non-admin users)
   
   **Usage:**
   - Without body: Scans all folders added via "Add Ebook Folder"
   - With folder_paths: Scans the specified paths directly (useful for testing)
+  
+  **Note:** All discovered ebooks will be associated with the admin user who initiated the scan.
 }