channels SWR follow-ups to #517: recover in background refresh, honour cache reset #519

Follow-ups from the #517 review ("Review — CS-pve-agent1 PR#517"). Fix these before the next release:

1. Recover in the background refresh (must fix). The stale-while-revalidate goroutine runs query() without recover. A panic in the query kills the whole server process instead of failing one request; the HTTP handler only recovers on the synchronous path. Add a recover that logs, keeps the stale entry and lets a later retry run. Add a test with a panicking query that proves the process survives and the stale entry is still served.
2. Honour reset() (nit). A refresh started before a cache reset, for example from the decode backfill, can finish afterwards and put its pre-reset result. Add a generation or epoch check so a refresh that started before a reset does not store its result. Test it.
3. Optional (info): re-check for a fresh entry before the background query, as the synchronous flight does.

Keep the #499 correctness tests and the #495 lock rule.