🛡️ Sentinel: [HIGH] Fix default bypasses and hardcoded secrets

[secrooq]

🚨 Severity: HIGH
💡 Vulnerability: Hardcoded admin bypass secret and Turnstile fallback keys could allow unauthenticated users or bots to bypass critical authentication and anti-spam verification when environment variables are unset.
🎯 Impact: If TURNSTILE_SECRET_KEY is omitted, the system defaults to a bypass ("1x...") permitting bots. Similarly, if ADMIN_BYPASS_SECRET is unset, the system defaulted to a hardcoded string ("stealth_dev_bypass_root_2026") that could be discovered, allowing unauthorized bypass of admin MFA in the edge API.
🔧 Fix:

• Explicitly set verifyTurnstile bypasses to rely strictly on non-production environment variables (process.env.NODE_ENV !== "production").
• Refactored verifyTurnstile to fail-closed (return false) when the TURNSTILE_SECRET_KEY is not present instead of using default "1x..." strings.
• Removed the hardcoded admin TOTP bypass in src/auth.ts. It now strictly checks process.env.ADMIN_BYPASS_SECRET and fails if it is omitted.
  ✅ Verification: Verified by running pnpm run build, pnpm test, and npx playwright test. Checked that linting passes (with previous acceptable application warnings ignored).

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PR created automatically by Jules for task 17185414317553584814 started by @secrooq

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