🛡️ Sentinel: [CRITICAL] Fix hardcoded NextAuth secret fallback

[secrooq]

🚨 Severity: CRITICAL
💡 Vulnerability: Found a hardcoded AUTH_SECRET fallback value ("stealth_relay_nextauth_secret_key_edge_v1_2026") used globally in NextAuth initializations within src/auth.ts and src/middleware.ts.
🎯 Impact: If the AUTH_SECRET environment variable was missing (e.g., misconfigured edge environment), the application would automatically fall back to the known, hardcoded secret. This severely compromises session integrity and allows attackers to forge valid authentication sessions across environments. NextAuth is designed to fail securely if secrets are not provided, but this fallback bypassed that protection.
🔧 Fix: Removed the hardcoded fallback assignment in both files. NextAuth will now strictly rely on the environment variable (getEnv("AUTH_SECRET")). In the event of a missing secret, the application will fail closed rather than failing open with a known public secret. Documented the learning in .jules/sentinel.md.
✅ Verification: Verified tests run successfully and the build completes without errors after updating the fallback logic. Unstaged lockfiles to keep the PR clean.

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PR created automatically by Jules for task 4719439984369879818 started by @secrooq

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