fix(worker): fence async operation writes by claim generation

[sireika]

Closes #2816.

Summary

• Carry each execution attempt's exact database claim identity, (worker_id, claimed_at), from claim readback through the worker task context.
• Fence completion, failure, retry, defer, and unknown-task deletion writes against that exact claim.
• Make dependent side effects conditional on winning the fence: parent aggregation, consolidation webhook enqueue, and the file-conversion-to-retain handoff.
• Normalize Oracle timestamp strings before binding claimed_at back into compare-and-swap queries.

Why claimed_at

worker_id alone is not a claim generation: a stable worker ID can reclaim an operation while an older coroutine from the same worker is still running. The claim path now reads back the exact database-stored claimed_at value after the claim update and uses it together with worker_id for subsequent compare-and-swap predicates.

This reuses existing columns, so there is no schema migration. It also does not change persisted task payloads, executor signatures, or public APIs. Direct/non-worker call paths retain their existing idempotency guards.

Safety properties

• A stale attempt cannot complete, fail, retry, defer, or delete a newer claim.
• Parent aggregation and consolidation webhook delivery are skipped after claim loss.
• Claimed-owner webhook enqueue is checked and inserted in one transaction while the operation row is locked.
• File conversion completion and downstream retain creation remain atomic; a stale conversion creates no child task and emits no completion hook.
• PostgreSQL and Oracle both return the exact stored claim timestamp; Oracle string timestamps are normalized before reuse.

Verification

• ./scripts/hooks/lint.sh
• 116 operation-completion and worker tests passed.
• 115 file-handoff, cancellation-compatibility, and database-adapter tests passed.
• Post-rebase critical regression run: 12 passed, including a real PostgreSQL same-worker reclaim case.
• Independent review: no remaining Critical or Important findings.