🛡️ Sentinel: [HIGH] Fix unsafe CORS OPTIONS routing

[jackby03]

🚨 Severity: HIGH
💡 Vulnerability: Unsafe CORS OPTIONS Routing. In Go 1.22's http.ServeMux, the OPTIONS /api/v1/auth/login route was explicitly mapped to s.handleLogin. While the previous CORS middleware happened to short-circuit this before execution, directly mapping a preflight request to a business-logic handler creates a dangerous attack vector if middleware configurations ever shift, potentially executing state-changing logic or exposing backend handlers.
🎯 Impact: Exposes login and potentially other sensitive handlers to unintended execution or unauthenticated probing via preflight requests.
🔧 Fix: Refactored corsMiddleware to use the standard http.Handler wrapper pattern. Applied this middleware globally to the entire ServeMux router rather than applying it per-route. Removed all explicit OPTIONS route definitions and stripped the legacy inline withCORS wrappers.
✅ Verification: Verified via go test ./... with CGo disabled (all tests pass) and visually validated the journal formatting in .jules/sentinel.md.

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PR created automatically by Jules for task 11514851127263169333 started by @jackby03

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