Add global registration disable feature

Implement configurable registration control to allow administrators to disable new user signups.

Backend changes:
- Add fitpub.registration.enabled configuration property (defaults to true)
- Update AuthController to check registration status and return 403 Forbidden when disabled
- Create GET /api/auth/registration-status endpoint to expose registration status to frontend
- Add RegistrationStatusResponse DTO

Configuration changes:
- Add REGISTRATION_ENABLED environment variable to application.yml
- Add REGISTRATION_ENABLED to Dockerfile with default value of true
- Update .env.example with REGISTRATION_ENABLED documentation

Frontend changes:
- Update registration page to check status and hide form when disabled
- Add checkRegistrationStatus() to auth.js to dynamically hide registration links
- Display user-friendly message when registration is disabled

To disable registration, set environment variable: REGISTRATION_ENABLED=false

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/main/resources/templates/auth/register.html

@@ -174,7 +174,7 @@
     <!-- Custom Scripts -->
     <th:block layout:fragment="scripts">
         <script th:inline="javascript">
-            document.addEventListener('DOMContentLoaded', function() {
+            document.addEventListener('DOMContentLoaded', async function() {
                 const form = document.getElementById('registerForm');
                 const registerBtn = document.getElementById('registerBtn');
                 const registerBtnText = document.getElementById('registerBtnText');
@@ -183,6 +183,23 @@
                 const successAlert = document.getElementById('successAlert');
                 const errorMessage = document.getElementById('errorMessage');
 
+                // Check if registration is enabled
+                try {
+                    const response = await fetch('/api/auth/registration-status');
+                    const data = await response.json();
+
+                    if (!data.enabled) {
+                        // Registration is disabled - show message and hide form
+                        errorMessage.textContent = 'Registration is currently disabled on this instance. Please contact the administrator if you need an account.';
+                        errorAlert.classList.remove('d-none');
+                        form.style.display = 'none';
+                        return;
+                    }
+                } catch (error) {
+                    console.error('Error checking registration status:', error);
+                    // Continue to show form if check fails
+                }
+
                 // Password confirmation validation
                 const password = document.getElementById('password');
                 const confirmPassword = document.getElementById('confirmPassword');