Add “Distributor” Rebate Category

Feedback from: Chris J

Add support for a new rebate category where the sponsor is Distributor with source Milestone.

Scope includes:

• Add new sponsor value: Distributor (source must be Milestone)

• Enable filtering via sponsor=distributor and source=milestone in Public API v1 & v2

• Update API validation and documentation

• Update PIM to include “Distributor” in Buying Group

• Restrict sponsor to “Milestone” (for now) when Buying Group = Distributor

Goal: Ensure distributor-direct rebates are properly supported, filterable, and exposed in both API and PIM.