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/application.yml

@@ -62,6 +62,10 @@ fitpub:
       secret: ${JWT_SECRET:change-this-secret-key-in-production-must-be-at-least-32-characters-long}
       expiration: 86400000 # 24 hours in milliseconds
 
+  # Registration settings
+  registration:
+    enabled: ${REGISTRATION_ENABLED:true}
+
   # Storage settings
   storage:
     fit-files: