Merchant daily performance uses inconsistent category numbering for empty vs populated results

[StuartFerguson]

BuildMerchantDailyPerformanceBaseMetrics still emits categories 0..7, while BuildMerchantDailyPerformanceMetrics now emits 1..5 plus the top-product metric.

Impact:

• The same endpoint returns different category contracts depending on whether any sales exist
• Consumers can no longer rely on a stable category mapping

Suggested fix:

• Keep the category scheme consistent between the empty and populated paths, or replace it with the new Type field everywhere.