🛡️ Sentinel: [HIGH] Fix insecure CORS preflight routing

[jackby03]

🚨 Severity: HIGH
💡 Vulnerability: Insecure CORS Preflight Handler Route Binding. The OPTIONS route for /api/v1/auth/login was explicitly mapped to the srv.handleLogin business logic handler.
🎯 Impact: Unauthenticated CORS preflight requests (OPTIONS) could improperly invoke or hit business logic meant only for authenticated/intended POST requests.
🔧 Fix: Refactored corsMiddleware from http.HandlerFunc to http.Handler and moved it to wrap the global ServeMux instance. Removed explicit OPTIONS route binding entirely.
✅ Verification: Ran go test ./cmd/... successfully. All CORS functionality still fully tested.

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PR created automatically by Jules for task 7666048950231908019 started by @jackby03

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