feat: Add public endpoint to get registration configuration

Backend Changes:
- Added GET /api/v1/auth/registration/config public endpoint
- Returns registrationMode and requireInviteCode settings
- No authentication required - accessible before registration
- Injected ConfigService into AuthController

This allows the frontend to dynamically show/hide the invite code
field based on the current registration mode setting.

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

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

maternal-web/app/(auth)/register/page.tsx

@@ -23,6 +23,7 @@ import * as z from 'zod';
 import { useAuth } from '@/lib/auth/AuthContext';
 import Link from 'next/link';
 import { useTheme } from '@mui/material/styles';
+import apiClient from '@/lib/api/client';
 
 const registerSchema = z.object({
   name: z.string().min(2, 'Name must be at least 2 characters'),
@@ -79,6 +80,8 @@ export default function RegisterPage() {
   const [isLoading, setIsLoading] = useState(false);
   const [userAge, setUserAge] = useState<number | null>(null);
   const [requiresParentalConsent, setRequiresParentalConsent] = useState(false);
+  const [requireInviteCode, setRequireInviteCode] = useState(false);
+  const [loadingConfig, setLoadingConfig] = useState(true);
   const { register: registerUser } = useAuth();
 
   const {
@@ -95,6 +98,26 @@ export default function RegisterPage() {
     },
   });
 
+  // Fetch registration configuration on mount
+  useEffect(() => {
+    const fetchRegistrationConfig = async () => {
+      try {
+        const response = await apiClient.get('/api/v1/auth/registration/config');
+        if (response.data?.success && response.data?.data) {
+          setRequireInviteCode(response.data.data.requireInviteCode);
+        }
+      } catch (error) {
+        console.error('Failed to fetch registration config:', error);
+        // Default to not requiring invite code if fetch fails
+        setRequireInviteCode(false);
+      } finally {
+        setLoadingConfig(false);
+      }
+    };
+
+    fetchRegistrationConfig();
+  }, []);
+
   // Watch date of birth to calculate age and show parental consent if needed
   const dateOfBirth = watch('dateOfBirth');
 
@@ -244,26 +267,30 @@ export default function RegisterPage() {
               }}
             />
 
-            <TextField
-              fullWidth
-              label="Invite Code (Optional)"
-              margin="normal"
-              error={!!errors.inviteCode}
-              helperText={errors.inviteCode?.message || 'Leave blank if registration is open'}
-              {...register('inviteCode')}
-              disabled={isLoading}
-              inputProps={{
-                'aria-invalid': !!errors.inviteCode,
-                'aria-describedby': errors.inviteCode ? 'invite-code-error' : 'invite-code-helper',
-              }}
-              InputProps={{
-                sx: { borderRadius: 3 },
-              }}
-              FormHelperTextProps={{
-                id: errors.inviteCode ? 'invite-code-error' : 'invite-code-helper',
-                role: errors.inviteCode ? 'alert' : undefined,
-              }}
-            />
+            {requireInviteCode && (
+              <TextField
+                fullWidth
+                label="Invite Code"
+                margin="normal"
+                error={!!errors.inviteCode}
+                helperText={errors.inviteCode?.message || 'Enter your invite code to register'}
+                {...register('inviteCode')}
+                disabled={isLoading}
+                required
+                inputProps={{
+                  'aria-required': 'true',
+                  'aria-invalid': !!errors.inviteCode,
+                  'aria-describedby': errors.inviteCode ? 'invite-code-error' : 'invite-code-helper',
+                }}
+                InputProps={{
+                  sx: { borderRadius: 3 },
+                }}
+                FormHelperTextProps={{
+                  id: errors.inviteCode ? 'invite-code-error' : 'invite-code-helper',
+                  role: errors.inviteCode ? 'alert' : undefined,
+                }}
+              />
+            )}
 
             <TextField
               fullWidth