[ACCOUNTING] Outgoing Keysend Callback

[joshr4]

• Determine all side effects and add to this issue
• Migrate legacy code to new accounting service controller
• Identify all existing code that uses the legacy endpoint and create a follow up ticket to update the call sites to use the new endpoint

[joshr4]

This callback route is called when an outgoing keysend payment is complete.

The controller logic looks up the keysend record in the external_payment table, marks it as is_pending false, and updates its status.

If the payment was successful, the sender's balance is adjusted accordingly. If unsuccessful, the sender's balance is left unchanged.

[joshr4]

ZBD is the only consumer of this endpoint, there are no clients to update.