feat(security): implement zeroize secrets pool and system.injectCredentials (#8)

[ZhuchkaTriplesix]

Summary of Changes

• Created ConnectionSecretsPool in src/utils/secret_guard.rs backed by secrecy::SecretString ( upon drop).
• Implemented system.injectCredentials RPC handler to safely inject passwords / JWT tokens into the memory pool without any disk persistence or logging.
• Hooked system.shutdown handler to call ConnectionSecretsPool::global().clear_all() to guarantee immediate zeroization of heap allocations before process exit.
• Added fast-track routing in src/main.rs for system.injectCredentials ( latency).
• Added 4 new unit tests ().

Closes #8