feat(security): add rate limiting to OAuth endpoints

- Install express-rate-limit dependency for protection against abuse
- Add comprehensive rate limiting configuration with JSON-RPC 2.0 error format
- Configure different limits: 100/15min for OAuth endpoints, 10/15min for token endpoint
- Include structured logging for rate limit violations
- Prepare for applying rate limits to sensitive OAuth routes

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: nickytonline

package-lock.json

@@ -42,6 +42,7 @@
     "@types/express": "^5.0.3",
     "@types/oauth2-server": "^3.0.18",
     "express": "^5.1.0",
+    "express-rate-limit": "^8.0.1",
     "jose": "^6.0.12",
     "oauth2-server": "^3.1.1",
     "pino": "^9.0.0",