refactor: Reorganize Bruno requests by role permissions

- Create separate admin folder for admin-only operations
- Move admin endpoints (Create/Update/Delete Ebook, Scanner, Folder Management) to /bruno/admin/
- Add admin dashboard, profile, and library page requests
- Add Register Admin User request with explicit admin role
- Update Register User request to include role field
- Remove empty scanner folder structure

API organization:
- Admin requests: /bruno/admin/ (require admin role)
- User requests: /bruno/user/ (available to all authenticated users)
- Environment: Single {{token}} variable works for both roles

bruno/scanner/Start Scanner.bru

@@ -1,39 +0,0 @@
-meta {
-  name: Start Scanner
-  type: http
-  seq: 2
-}
-
-post {
-  url: {{base_url}}/api/scanner/start
-  body: json
-  auth: inherit
-}
-
-settings {
-  encodeUrl: true
-  timeout: 0
-}
-
-docs {
-  ## Start Scanner
-  
-  Starts monitoring the specified folders for ebook changes.
-  
-  **Method:** POST
-  
-  **Endpoint:** /api/scanner/start
-  
-  **Authentication:** Required
-  
-  **Request Body:**
-  - `folder_paths` (array of strings, required): Array of folder paths to monitor
-  
-  **Response:**
-  - `message` (string): Success message
-  
-  **Status Codes:**
-  - 200: Success
-  - 400: Bad Request
-  - 401: Unauthorized
-}