Add structured log for each successful creator key buy including buyer wallet, creator, amount, and new supply

[Chucks1093]

Summary

Buy transactions are the core activity of the AccessLayer marketplace but are not logged at a structured level. An info-level log on each confirmed buy would give operators full visibility into marketplace volume and help correlate on-chain events with server activity.

Scope

• Emit an info-level log on each confirmed buy with fields: buyer_wallet, creator_wallet, key_amount, xlm_paid, new_supply, tx_hash, confirmed_at
• Log emitted after the transaction is confirmed on-chain, not on submission
• xlm_paid should be the net amount including fees, formatted as XLM (not stroops)
• Do not log private key material or signing secrets

Acceptance Criteria

• Info log emitted on each confirmed buy with all seven fields
• Log emitted after on-chain confirmation, not submission
• xlm_paid formatted as XLM string (not stroops)
• No key material or secrets in the log

ETA: 12 hours

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Coordinate on Telegram

[Mona-i]

Hey

I would love to work on this issue.