🛡️ Sentinel: [HIGH] Fix CORS preflight vulnerability

[jackby03]

🚨 Severity: HIGH
💡 Vulnerability: Missing CORS middleware on unauthenticated endpoints, and exposing business-logic endpoints to unauthenticated preflight OPTIONS requests.
🎯 Impact: Attackers could send unauthorized cross-origin requests, or bypass preflight checks on sensitive endpoints, exposing the API to cross-origin abuse.
🔧 Fix: Refactored corsMiddleware in internal/api/middleware.go to accept an http.Handler instead of http.HandlerFunc, and wrapped the global server mux in internal/api/server.go. Removed individual endpoint withCORS wrappers and the explicit unauthenticated OPTIONS route in internal/api/routes.go.
✅ Verification: Ran go test ./... with CGO_ENABLED=0 to ensure no functionality is broken and confirmed the middleware changes.

---

PR created automatically by Jules for task 6171335962296247257 started by @jackby03

[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.