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

[Magnopiro-oficial]

🚨 Severity: HIGH
💡 Vulnerability: API endpoints were unprotected against excessive requests (DoS/Brute Force) because rate limiting was commented out. Additionally, the backend was crashing on startup due to a Fastify version mismatch.
🎯 Impact: Attackers could flood the API with requests, causing denial of service or brute-forcing authentication.
🔧 Fix:

1. Downgraded @fastify/jwt to v8.0.1 (compatible with Fastify v4).
2. Enabled @fastify/rate-limit in app.ts using environment configuration.
3. Updated error.middleware.ts to handle numeric 429 error codes from the rate limiter.
   ✅ Verification: Added backend/src/__tests__/rate_limit.test.ts which confirms that excessive requests (over limit) are blocked with status 429.

---

PR created automatically by Jules for task 12238560872752442089 started by @Magnopiro-oficial

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.