Prevent stale workers from acknowledging newer message claims

[polyrand]

Context

LiteQueue can retry a locked message, but a claim has no identity beyond message_id. done(), mark_failed(), and retry() update any matching row without checking its current state.

Current code:

• pop() changes READY to LOCKED and returns the message.
• retry() changes any matching message to READY and leaves its old lock_time in place.
• done() and mark_failed() accept acknowledgements from any worker that knows the message ID.

A focused reproduction on commit 2b82ede showed this race:

1. Worker A pops a message.
2. Recovery retries the message.
3. Worker B pops the same message.
4. Worker A calls done(message_id).
5. LiteQueue marks worker B's active claim as done.

The stale done() returned True, and the newer claim ended in DONE.

Why implement this

Retry is meant to recover work after a worker stops responding. Without claim ownership, a slow worker can acknowledge a newer delivery. If the newer worker then fails, LiteQueue has already removed the task from active work. This breaks reliable redelivery and can lose work.

Proposed direction

• Add a claim token or monotonically increasing attempt number to each successful pop.
• Include the claim identity in the returned Message or a dedicated claim type.
• Require message_id, claim identity, and LOCKED status in done() and mark_failed() updates.
• Restrict retry to documented source states.
• Clear obsolete lock_time and done_time fields when returning work to READY.
• Provide one atomic operation for retrying expired claims instead of requiring list_locked() followed by separate retry() calls.
• Document and migrate the schema and acknowledgement API.

Acceptance criteria

• A stale worker cannot complete or fail a newer claim.
• Invalid transitions return a clear failure without changing the row.
• Retrying a claim clears obsolete timestamps.
• Expired-claim recovery is atomic under concurrent workers.
• Tests reproduce the stale-worker race for both pop implementations.
• make test passes.