Timer-driven library tasks fire on every replica, so a library refresh or a
database optimise runs once per pod against the same shared library.
- add IScanLeaderLease with a Redis TTL implementation and a no-op default
- skip timer-driven runs of the gated tasks on instances without the lease
- treat an unreachable Redis as holding the lease so tasks never stop running
- leave manual and API-triggered runs ungated
unkin-agent
3b416c7fb2