Every fee on the list points at the transfer it came from

Finance closes the month and asks what the $1.25 line is.

GET /wallet/ai-fee-history returns every platform fee charged on an AI credit transfer, each paired with the transaction it was charged against: the fee's amount and timestamp, then the transaction's id, its reason: ai_credit_topup: and its gross amount. The $1.25 sits next to the $25.00 it came from.

For the auditor, POST /wallet/invoices/generate/{transaction_id} produces the invoice for that transaction. If one already exists, the same id comes back with a 200; otherwise a new one is created with a 201. Run it twice and you have one invoice, not two. GET /wallet/invoices/{id}/pdf is the file.

The infrastructure side stays flat: one server, one monthly price, bandwidth and storage inside it. The AI side is a list of transfers. Every fee on the list points at the transfer it came from.