feat: schedule atproto crossposting with oldest-first backlog replay

[rabble]

Summary

• queue live relay events before cursor advance and publish them through dedicated live and backfill worker lanes
• seed migrated-user backlog as oldest-first backfill jobs while still allowing new live posts to publish immediately
• preserve delete-before-publish cancellation, add runtime scheduler coverage, and document the new rollout and ops behavior

Plan

• Implements Task 5 and Task 6 from docs/superpowers/plans/2026-03-29-oldest-first-crosspost-scheduler.md

Test Plan

• cargo test -p divine-atbridge --test publish_queue_scheduler -- --nocapture
• cargo test -p divine-atbridge --test backfill_planner -- --nocapture
• cargo test -p divine-atbridge --test publish_path_integration -- --nocapture
• cargo test -p divine-atbridge --test replay_and_delete -- --nocapture
• cargo test -p divine-atbridge --test runtime_resilience -- --nocapture
• cargo test -p divine-atbridge --test e2e_local_stack -- --nocapture
• cargo check -p divine-atbridge -p divine-bridge-db -p divine-bridge-types
• cargo check --workspace (still blocked by unrelated divine-feedgen missing skeleton::FeedStore / DbFeedStore)