[API] Strengthen request authentication and proxy trust validation

[Flegma]

Summary

Request authentication middleware and proxy trust configuration need hardening to ensure proper identity verification.

Tasks

• Add robust null/format checks in authentication middleware
• Configure proxy trust validation to only accept known proxies
• Remove TODO placeholder in trust proxy configuration

Impact

Could allow requests to bypass authentication or spoof client identity.

Details

Full details in internal audit document. Finding IDs: SEC-API-01, SEC-API-04

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Related Issues (Security Hardening Pattern)

• [Infrastructure] Tighten RBAC cluster roles & DaemonSet security context #412 — [Infrastructure] Tighten RBAC & DaemonSet security
• [Infrastructure] Add network policies & scope Vault permissions per service #413 — [Infrastructure] Network policies & Vault scoping
• [Connector] Improve offline match endpoint authentication #409 — [Connector] Improve offline match authentication
• [API] Strengthen request authentication and proxy trust validation #372 — [API] Strengthen request authentication