`next_cursor` is returned even for offset pagination, with unclear semantics

[Darkdruce]

Summary

The offset branch also returns a next_cursor derived from the last row, mixing two pagination models in one response.

Location

• src/api/payments.rs:276-284

Details & impact

Clients may interleave offset and cursor, producing inconsistent/duplicated pages since the two orderings differ (created_at DESC vs created_at DESC, id DESC).

Suggested fix

Pick one model per request and document that mixing is unsupported; ideally deprecate offset.

Acceptance criteria

• Pagination semantics are unambiguous and documented.