keeper: graceful shutdown and in-flight round draining in watch mode

[karagozemin]

Summary

services/keeper watch mode should handle SIGINT/SIGTERM by finishing any in-flight reveal/settle operation before exiting, instead of terminating mid-transaction.

Motivation

Killing the keeper mid-settle can leave a round in an ambiguous state and require manual recovery.

Acceptance criteria

• Signal handler that stops picking up new rounds and waits for the current one to complete (with a timeout).
• Structured log line on shutdown start/finish.
• Test simulating a shutdown signal during a queued round.
• README note on operational behavior.

[SHEROSE0]

Hi Maintainer,

I've read through the task description and understand the requirements. I'm confident I can work on this issue. Could you kindly assign it to me?

Thank you!

[samjay8]

Hello Maintainer, I have throughly checked this issue and I would like to contribute to your work by solving the issue, please kindly assign it to me.