Problem

A job acquires a 20-second Redis lease at 0 seconds and finishes at 1 second.
The renewal goroutine is already waiting for the first tick at about 6.7 seconds.
Previously, Unlock() stopped the ticker without closing its channel, leaving that goroutine blocked forever.
The explicit shutdown signal now wakes it so it can exit.

Summary

• End the long-operation renewal goroutine when Unlock() closes its shutdown channel. The goroutine stops its own ticker when it exits.
• Serialize renewal with unlock and check the current acquisition so a ready tick cannot renew a released or reused lock. Add regression checks for goroutine exit, lock reuse, and a tick arriving during unlock.
• Return the existing mutex directly for in-memory long operations, since those locks never expire and need no renewal goroutine.