feat(backend): implement issues #761, #762, #763, #764

Issue #761: Enhanced error recovery for Database Pooler
- Added circuit breaker pattern to prevent cascading failures
- Implemented health check function with connectivity testing
- Added pool warming on startup to reduce cold start latency
- Enhanced error logging with structured context
- Circuit breaker states: CLOSED, OPEN, HALF_OPEN with auto-recovery

Issue #762: Conducted security audit on Database Pooler
- Created comprehensive security audit documentation
- Verified connection string security
- Confirmed appropriate connection limits and timeouts
- Validated SSL/TLS configuration
- Assessed error handling and information disclosure
- Security rating: SECURE - No critical vulnerabilities

Issue #763: Implemented rate limiting for API Gateway Security
- Token bucket rate limiter with Redis backend
- Multiple rate limit tiers (per IP, per API key, per endpoint)
- Standard HTTP headers (X-RateLimit-*, Retry-After)
- Graceful degradation (fail open on Redis errors)
- Configurable capacity and refill rates

Issue #764: Cryptographic signature verification (Already implemented)
- Verified existing HMAC-SHA256 implementation
- Added key rotation support for zero-downtime updates
- Enhanced logging for signature verification
- Confirmed timing-safe comparison and replay protection

Closes #761
Closes #762
Closes #763
Closes #764

Author: Temi-suwa18