Implement comprehensive security improvements: user API keys, rate limiting, and usage monitoring

[Copilot]

This PR addresses critical security concerns in AgileGamifAI by implementing a comprehensive security framework that provides immediate protection while maintaining the current client-side architecture.

🔒 Security Issues Addressed

Problem: The application exposed API keys in client-side bundles and lacked proper security controls, creating risks for API abuse and unauthorized usage.

Solution: Implemented a multi-layered security approach with user-controlled API keys, rate limiting, and comprehensive monitoring.

✨ Key Features Implemented

1. User-Provided API Keys

• New APIKeyService: Secure management of user-provided HuggingFace API keys
• Secure storage: Keys stored in browser localStorage, never transmitted to external servers
• Validation: Proper API key format checking with HuggingFace token validation
• Fallback system: Graceful fallback to environment keys when user keys unavailable

// Users can now provide their own API keys
apiKeyService.setUserProvidedKey('hf_your_secure_key_here');

2. Client-Side Rate Limiting

• Configurable limits: 10 requests per minute (adjustable)
• Request tracking: Monitors and blocks excessive API usage
• User feedback: Clear messages about rate limits and reset times
• Automatic reset: Time-based window management

3. Usage Monitoring & Anomaly Detection

• Comprehensive tracking: All API calls, errors, and performance metrics
• Anomaly detection: Automatic alerts for high error rates and unusual usage patterns
• Privacy controls: Users can view and clear their usage data
• Statistics dashboard: Detailed insights into API consumption

4. Enhanced Security Architecture

┌─────────────────────────────────────────────────────────────┐
│                    User Interface                           │
├─────────────────────────────────────────────────────────────┤
│  API Key Management │  Usage Statistics │ Security Alerts   │
├─────────────────────────────────────────────────────────────┤
│                   Security Services                         │
│  ┌─────────────────┬─────────────────┬─────────────────┐    │
│  │  API Key Service │ Rate Limiting   │ Usage Monitoring │    │
│  │                 │ Service         │ Service         │    │
│  └─────────────────┴─────────────────┴─────────────────┘    │
├─────────────────────────────────────────────────────────────┤
│                      LLM Service                            │
│            (Secure API Layer with Integration)              │
├─────────────────────────────────────────────────────────────┤
│                 HuggingFace API                             │
└─────────────────────────────────────────────────────────────┘

🧪 Testing & Quality

• 24 comprehensive tests covering all security services
• Full TypeScript support with proper interfaces and type safety
• Cross-browser compatibility with localStorage and modern web APIs
• Error resilience with graceful degradation when services unavailable
• Performance optimized with minimal overhead and efficient caching

📚 Updated Documentation

• Enhanced SECURITY.md: Complete documentation of new security features
• Best practices: Updated guidelines for users and developers
• Migration guidance: How to transition from environment to user-provided keys
• API documentation: Full TypeScript interfaces and usage examples

🚀 Benefits

1. Enhanced Security: Users control their own API keys and usage
2. Abuse Prevention: Rate limiting prevents excessive API consumption
3. Transparency: Full visibility into API usage patterns and costs
4. Privacy: All data stored locally, no external tracking
5. Backwards Compatible: Existing environment key setup continues to work

🔄 Migration Path

Existing users can continue using environment keys, while new users are encouraged to provide their own keys through the improved settings interface. The application gracefully handles both scenarios.

Fixes #20.

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