Truncate pubkey in Lightning invoice creation logs

[Copilot]

Addresses privacy feedback from #183: the full 64-character pubkey was being logged in the Lightning invoice creation endpoint, enabling potential user correlation in production logs.

Changed log statement to truncate pubkey to 16-character prefix:

console.log('[Membership Lightning] Invoice created:', {
  receiveRequestId,
  amountSats,
  tier,
  period,
  pubkey: pubkey.substring(0, 16) + '...',  // was: pubkey
});

This follows the established pattern used in 50+ other locations throughout the codebase for logging user identifiers.

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