health: tolerate a transient receive-listener lag, fail only if it's stuck

The listener-backlog probe fired a failure whenever a paid invoice existed
beyond the durable cursor — but that is almost always the listener mid-
processing that very payment, a normal sub-second lag, not a wedged listener.
Those transient catches produced spurious "listener backlog" health failures
that then recovered on their own.

Grace-check instead: on detecting a paid invoice beyond the cursor, wait
briefly and re-read the cursor; if it advanced, the listener is alive and this
was transient (pass). Only fail if the cursor did not move at all — the actual
"stuck listener" condition the probe exists to catch.

Co-Authored-By: Claude Opus 4.8 <[email protected]>
Claude-Session: https://claude.ai/code/session_01DKkev76U4YRnRAqM6jqbae

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