🛡️ Sentinel: [HIGH] Enable rate limiting and fix JWT dependency

[Magnopiro-oficial]

🚨 Severity: HIGH
💡 Vulnerability: Missing rate limiting on API endpoints allowed potential DoS and brute force attacks.
🎯 Impact: Attackers could spam endpoints like /auth/nonce or /auth/login, exhausting resources or flooding Redis.
🔧 Fix:

1. Enabled @fastify/rate-limit in backend/src/app.ts.
2. Updated global error handler to map 429 status to RATE_LIMIT_EXCEEDED code.
3. Downgraded @fastify/jwt to ^8.0.0 to resolve incompatibility with Fastify v4 (was ^10.0.0 which requires v5).
4. Added test infrastructure (Jest config) and a test case rate_limit.test.ts.
   ✅ Verification:
   Run npm test in backend/ directory. Verified that /health endpoint returns 429 after limit is exceeded.

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PR created automatically by Jules for task 12730640235599005345 started by @Magnopiro-oficial

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