v1.5.0 — durable task queue

Durable, lease-based task queue (hallpass.taskqueue). TaskQueue on SQLite: claim hands one worker exactly one task under a write lock (no double-runs), an expired lease reclaims an abandoned task (dead-worker recovery), complete is idempotent by id, and results + backlog survive a restart. Crash-recovery + exactly-once, the two gaps the harness research flagged.