feat: add CORS support and create frontend integration example

[Pertempto]

Description

Add Cross-Origin Resource Sharing (CORS) support to enable browser-based clients and create a simple frontend example demonstrating the complete API integration. This completes the simple-sync MVP by making it accessible from web applications.

Acceptance Criteria

• Add CORS middleware to allow cross-origin requests
• Configure appropriate CORS headers for development and production
• Create simple HTML/JavaScript frontend example
• Demonstrate authentication flow (login, get token)
• Show event retrieval and posting from browser
• Include ACL management functionality in example
• Provide clear error handling and user feedback
• Document frontend integration patterns

Technical Requirements

• Use Gin CORS middleware or implement custom CORS handler
• Allow appropriate HTTP methods (GET, POST, PUT, OPTIONS)
• Set proper Access-Control headers for development
• Frontend should be vanilla HTML/CSS/JavaScript (no frameworks)
• Include form-based login and event management
• Show real-time event updates after posting
• Handle authentication errors gracefully
• Provide example of token storage and usage

CORS Configuration

Access-Control-Allow-Origin: * (or specific domains)
Access-Control-Allow-Methods: GET, POST, PUT, OPTIONS
Access-Control-Allow-Headers: Content-Type, Authorization
Access-Control-Allow-Credentials: true

Frontend Example Features

• Login form with username/password
• Display current events with filtering
• Add new events form
• ACL management interface
• Token expiration handling
• Error message display
• Responsive design for mobile/desktop

Definition of Done

• CORS is properly configured and working
• Browser can make authenticated requests to all endpoints
• Frontend example successfully authenticates users
• Events can be retrieved and posted from browser
• ACL permissions can be managed through frontend
• Example includes proper error handling
• Documentation shows how to integrate with other frontend frameworks
• CORS settings are production-ready

Dependencies

• Requires completion of all previous MVP features
• Should work with Docker deployment setup
• Frontend should work with the deployed service