🛡️ Sentinel: [HIGH] Enable rate limiting and fix error handling

[Magnopiro-oficial]

🚨 Severity: HIGH
💡 Vulnerability: Missing Rate Limiting on sensitive endpoints. The rate limiting middleware was present but commented out in backend/src/app.ts, exposing the application to DoS and brute force attacks.
🎯 Impact: Attackers could flood the server with requests, causing denial of service, or brute force sensitive endpoints.
🔧 Fix:

1. Enabled @fastify/rate-limit in backend/src/app.ts using existing configuration.
2. Updated backend/src/shared/middleware/error.middleware.ts to correctly handle rate limit errors (code 429) and preserve the status code.
3. Downgraded @fastify/jwt to v8.0.1 to resolve a version mismatch with fastify v4.x that was preventing the application from starting during tests (v10+ targets Fastify v5).
4. Exposed app.initialize() as public to facilitate integration testing.
   ✅ Verification:

• Created backend/src/__tests__/rate_limit.test.ts to verify that requests exceeding the limit receive a 429 status code and the correct error payload.
• Verified that the global error handler wraps the rate limit error correctly (success: false).

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PR created automatically by Jules for task 15017004172893238930 started by @Magnopiro-oficial

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