payments: normalize bolt11 to lowercase so failed uppercase sends get reconciled

BOLT11 is case-insensitive, but an uppercase-pasted invoice was stored as
the payment hash and the pending-set entry verbatim. check()'s
startsWith("ln") test is case-sensitive, so it treated such pending
entries as non-lightning and evicted them without reversing — a failed
send would never be auto-refunded.

- sendLightning: lowercase the invoice up front so the stored hash, the
  pending entry, and xpay all use the same form.
- check(): lowercase before the startsWith("ln") test (defensive; also
  rescues any already-stored uppercase entries).

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

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