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>

.env.example

@@ -18,6 +18,10 @@ APP_BASE_URL=https://example.com
 JWT_SECRET=change_me_to_a_secure_random_string_in_production
 JWT_EXPIRATION_MS=86400000
 
+# Registration Configuration
+# Set to false to disable user registration
+REGISTRATION_ENABLED=true
+
 # ActivityPub Configuration
 ACTIVITYPUB_ENABLED=true