[WIP] Prepare #999 partial payout requests PR

[veracindarella]

Status

This is a draft tracking PR for #999. It contains no application, schema, or test changes; partial payout support is not implemented by this PR.

Intended scope

• Accept and validate a caller-specified payout amount against minimum and available balance.
• Reserve only the requested amount, leaving the remainder available.
• Make the balance check and reservation safe against concurrent requests using an appropriate database transaction/locking strategy.
• Add tests for partial balances, insufficient/minimum amounts, concurrency, and document the request/response behavior in Swagger.

Validation

Not run: this PR has no implementation changes.

Fixes #999