praisonai-platform: JWT signing key defaults to hardcoded "dev-secret-change-me", allowing token forgery for any user when PLATFORM_ENV is unset

Summary Type: Insecure default cryptographic key. The JWT signing secret defaults to the hardcoded literal "dev-secret-change-me" when PLATFORMJWTSECRET is unset. A safety check exists but only fires when PLATFORMENV != "dev"; the default value of PLATFORMENV is "dev", so the check is silently...