[Thom Chiovoloni (thomcc-work)]

Summary

Prevent a backlog of deferred activities from starving unrelated ready work.

Previously, worker fetches selected eligible rows by queue ID alone. Deferred
items retained their original IDs, so if their retry delays expired before the
backlog drained, they could repeatedly be selected ahead of newer ready work.

Migration 0027 changes both session-aware and non-session fetches to order
eligible rows by visible_at, id. A deferred item therefore goes behind work
that was already ready, even after its delay expires.

• Add a matching index on worker_queue(visible_at, id).
• Preserve session ownership, tag filtering, peek-lock semantics, and attempt accounting.
• Add regression tests covering expired deferrals across session/non-session and tagged/untagged work.
• Update the changelog and migration guidance.

This is the PostgreSQL-provider companion to microsoft/duroxide#74. It does
not change activity admission APIs or introduce a Duroxide dependency bump.

Migration and compatibility

• Replaces fetch_work_item without changing its signature or return shape.
• Builds the new index non-concurrently, blocking queue writes during the build.
  Plan migration time accordingly for large queues.
• Already-running older providers can use the updated function. Older binaries
  reject the new migration version on startup, so coordinate migration and
  worker rollout.