🛡️ Sentinel: [HIGH] Fix hardcoded HMAC secret in webhook signing

[Pmaster-dev]

🚨 Severity: HIGH
💡 Vulnerability: Hardcoded HMAC secret salt ("idsec_secret_salt") used to generate SHA-256 signatures for webhook payloads.
🎯 Impact: Attackers could forge valid HMAC signatures and bypass payload authenticity verification.
🔧 Fix: Replaced hardcoded secret with process.env.WEBHOOK_SECRET || process.env.JWT_SECRET and added fail-safe validation.
✅ Verification: Code review verified and read_file confirmed clean diff under 50 lines.

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PR created automatically by Jules for task 4231734867711835847 started by @Pmaster-dev

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