payments: prevent same-invoice retry from orphaning an unreconciled debit

The pending reconciliation set is keyed by bolt11, shared across attempts
at the same invoice. A retry made after a prior attempt's CLN parts
resolved (but before check() reconciled its debit) would pass the
listpays-only guard, and the retry's own reverse()/finalize() then
removed the shared entry — permanently hiding the first attempt's debit
from check(), so a failed send was never auto-refunded.

- sendLightning: refuse to pay while the invoice's bolt11 is still in
  the pending set; the caller is told to retry shortly.
- check(): an entry whose payment record no longer exists in db or
  archive is dropped from the set instead of skipped, so a dead entry
  can't block its invoice forever under the new guard.

Co-Authored-By: Claude Fable 5 <[email protected]>

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