🛡️ Sentinel: [CRITICAL] Fix Device JWT Subject Spoofing (Privilege Escalation)

[ToolchainLab]

🚨 Severity: CRITICAL
💡 Vulnerability: Device-signed JWTs trusted the sub claim without verifying it matched the device owner's user_id, allowing an attacker with one valid device key to impersonate any user.
🎯 Impact: Privilege escalation / full account takeover. A valid user could forge tokens for admins or other users.
🔧 Fix: Added explicit enforcement of claims.sub == device.user_id during JWT verification in verify_device_jwt.
✅ Verification: Ran uv run pytest. The test suite confirmed valid tokens pass and test_exploit.py confirmed spoofed tokens fail with Subject mismatch.

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PR created automatically by Jules for task 3279532018961842619 started by @ToolchainLab

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