feat(api-gateway): implement enterprise-grade gateway with tiered access, rate limiting, and SLA monitoring

[phertyameen]

feat(api-gateway): implement enterprise-grade gateway with tiered access, advanced rate limiting, usage analytics, and SLA enforcement

• Introduced API gateway enhancements for enterprise-grade performance and control
• Implemented tiered access levels to support multiple service plans
• Added advanced rate limiting with support for burst handling
• Integrated real-time usage analytics via monitoring module
• Enforced SLA management with performance and uptime tracking
• Added billing hooks for usage-based invoicing and reporting
• Optimized API throughput and request handling for high concurrency

Affected Modules:

• src/api-security/: enhanced request routing, access control
• src/rate-limit/: added burst handling and tier-aware limits
• src/auth/: implemented tiered user authentication/authorization
• src/monitoring/: real-time usage tracking and alerting hooks

Closes #130