🛡️ Sentinel: Fix insecure webhook signature generation

[Pmaster-dev]

🚨 Severity: HIGH
💡 Vulnerability: Webhook payloads were using base64 string encoding for the X-Webhook-Signature header rather than a cryptographic HMAC signature, allowing third parties to forge or tamper with webhook signatures.
🎯 Impact: Attackers could forge valid-looking webhook signatures or modify payload content in transit.
🔧 Fix: Replaced base64 pseudo-signature with an HMAC-SHA256 signature generated over the timestamp and payload using crypto.createHmac and WEBHOOK_SECRET / SESSION_SECRET.
✅ Verification: Tested signature generation using Node.js crypto via tsx and verified output format t=,v1=.

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PR created automatically by Jules for task 16997198982599385288 started by @Pmaster-dev

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