🛡️ Sentinel: [CRITICAL] Fix hardcoded secrets

[secrooq]

🚨 Severity: CRITICAL
💡 Vulnerability: Hardcoded fallback secrets for AUTH_SECRET and ADMIN_BYPASS_SECRET were found in src/auth.ts and src/middleware.ts.
🎯 Impact: If the environment variables were missing, the application would silently fallback to known keys ("stealth_dev_bypass_root_2026" and "stealth_relay_nextauth_secret_key_edge_v1_2026"), providing attackers with a known backdoor and decryption key.
🔧 Fix: Removed the hardcoded fallbacks and updated the code to require the explicit environment variables or fail securely.
✅ Verification: Ran pnpm test, pnpm run build, and npx playwright test to ensure changes pass successfully.

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PR created automatically by Jules for task 3448338147996854965 started by @secrooq

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