server: harden lightning payment-attempt settlement against optimistic-lock races

Kobe Nysrequested to merge
attempt-settlement into master

Summary by CodeRabbit

  • Bug Fixes
    • Improved payment settlement reliability by preventing rare race conditions from leaving linked Lightning spend records in the wrong state.
    • Linked payment-related outputs are now only marked as spent after the payment update is confirmed, reducing the chance of inconsistent status updates.
    • Settled payments are now handled more consistently even when updates happen concurrently.
Edited by coderabbit

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