Add count_unclaimed_ready_tasks function

[Aaron1011]

We can use this to monitor our running autopilot queue, and alarm if a large number of tasks are persistently unclaimed

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Note

Low Risk
Additive SQL function and client wrapper with new tests; low risk aside from potential performance impact of the count query on large queues.

Overview
Adds a new Postgres function durable.count_unclaimed_ready_tasks(queue) that returns the number of runs that are ready to be claimed (matching claim_task candidate eligibility but without locking/updating).

Exposes this via a new Rust client method Durable::count_unclaimed_ready_tasks, and includes a migration plus integration tests covering empty queues, multiple queues, worker-claim depletion, and excluding runs scheduled in the future.

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