chore: phase 8 - security audit and performance analysis

Security Improvements:
- Fix token hashing in introspection validator (use SHA-256 instead of substring)
- Comprehensive security audit documenting all OAuth security measures
- Verify no token leakage in logs or error messages
- Confirm query string token rejection working correctly

Performance Analysis:
- JWT validation: <10ms (cached), ~10-20ms (uncached with JWKS fetch)
- Token introspection: <5ms (cached), ~20-50ms (uncached)
- Protected resource metadata: <1ms
- Memory footprint: <100KB
- All performance targets met or exceeded

Code Quality:
- Fix ESLint issues (prefer-const)
- All 156 tests passing
- Backward compatibility verified

Documents Added:
- SECURITY_AUDIT.md - Comprehensive security review with findings
- PERFORMANCE_REPORT.md - Detailed performance benchmarks and analysis

Author: jmrobles