feat: Implement role-based registration restrictions and reorganize Bruno collection

- Add role-based restrictions to POST /api/auth/register endpoint
- Only admins can create admin accounts if any admin already exists
- First user automatically gets admin role regardless of request
- Regular users can only create user accounts, not admin accounts
- Unauthenticated users can only create first admin, not subsequent admins
- Reorganize Bruno collection into logical subfolders (auth/, admin/, profile/)
- Update documentation to reflect new registration restrictions and security rules

BREAKING CHANGES:
- /api/auth/register now enforces role-based creation restrictions
- Bruno collection reorganized with subfolder structure

bruno/user/profile/Get Profile.bru

@@ -0,0 +1,40 @@
+meta {
+  name: Get Profile
+  type: http
+  seq: 1
+}
+
+get {
+  url: {{base_url}}/api/auth/profile
+  body: none
+  auth: inherit
+}
+
+settings {
+  encodeUrl: true
+  timeout: 0
+}
+
+docs {
+  ## Get User Profile
+  
+  Retrieves the authenticated user's profile.
+  
+  **Method:** GET
+  
+  **Endpoint:** /api/auth/profile
+  
+  **Authentication:** Required
+  
+   **Response:**
+   - `id` (string): User ID
+   - `email` (string): Email
+   - `username` (string): Username
+   - `theme` (string): User theme preference
+   - `first_name` (string, optional): First name
+   - `last_name` (string, optional): Last name
+  
+  **Status Codes:**
+  - 200: Success
+  - 401: Unauthorized
+}