🔒 fix: secure /health endpoint against unauthorized access

[max-ostapenko]

🎯 What: Secured the /health endpoint by restricting access. It now returns 403 Forbidden unless the request originates from localhost (127.0.0.1, ::1) or contains a GoogleHC User-Agent (used by Cloud Run infrastructure probes).

⚠️ Risk: While a /health endpoint is generally lightweight, leaving it unauthenticated and open allows potential attackers to perform service discovery and map the service's footprint. Additionally, the previous unverified setup could be more vulnerable to Denial-of-Service attacks if the endpoint performs more intensive background checks.

🛡️ Solution: Added a security check in index.js that inspects the remote address and User-Agent. It allows legitimate local Docker health checks and Cloud Run platform probes, but denies external traffic. Removed insecure logic that attempted to parse client IPs from the spoofable X-Forwarded-For header.

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PR created automatically by Jules for task 11187011909452902925 started by @max-ostapenko

[google-labs-jules]

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