[Offers/Invoices] service_type is ignored on input - documentation unclear

[zeitpol]

The service_type field on offer/invoice items appears to be ignored when creating or updating
offers and invoices via the API. The documentation suggests it should be settable, but testing
shows it's always overwritten to "service" regardless of input.

The same behavior occurs for invoices - service_type is always returned as "service" regardless of
input.

Expected Behavior

Either:

1. The API should accept service_type as input and respect the value, OR
2. The documentation should clarify that service_type is output-only and explain how to create
   expense-type items (e.g., via revenue_category_id or linking to project expenses)

Questions

1. Is there a way to set service_type: "expense" on offer items via the API?
2. Does revenue_category_id control this? If so, how do we list available revenue categories?
3. Should the documentation be updated to clarify this is output-only?