Module pool 

Background metrics polling pool.

Each host gets its own persistent tokio task that manages its SSH connection and collects metrics at a configurable interval.

Architecture:

• PollManager — created by the main app, owns abort handles.
• One HostPoller task per host — loops indefinitely until aborted.
• Implements exponential backoff on connection failures.
• One SSH connection per host, reused across polls.
• All data sent to the main event loop via mpsc::Sender<CoreEvent>.

Structs

PollManager

Manages background metric polling for all hosts.