List Organization Pricing

List the organization's rate overrides.

Readable by any member: these rates decide what the caller's own requests cost, so they are not withheld from the people billed at them. Writing needs an owner or admin.

Paged on the same bounds the rest of the tenancy surface uses, because the table grows a row per model per period. count is the total, so a client knows whether another page is owed.

model_key narrows to one model, which is what an editor for that model needs: every period stored for it, so it can open on the one in force and refuse a new one that would overlap. Normalized the same way a write is, so a legacy provider/model spelling finds the rows a canonical one stored.