Use UNLINK in cancel_all, document leased-timer semantics, plumb client through claim

[lesnik512]

• O4: cancel_all now uses UNLINK instead of DEL so freeing large topic keys runs in Redis's background thread (Redis 4.0+).
• S1: get_pending_timers docstring spells out that leased (in-flight) timers appear in the default call but are excluded once before is set to "now".
• S2: cancel_all docstring notes that handlers already executing for a leased timer continue to run to completion; their final commit is a no-op because the keys are gone.
• B6: _claim_and_consume now receives client via tg.start_soon instead of resolving self._client, so the whole subscriber poll-and-claim path uses one consistent client reference.