Refactor customers mart rollup

[mirnawong1]

Summary

• drive the customers rollup from the customer entity table
• use left joins so the customers mart preserves one row per customer
• keep existing customer lifetime metric columns and customer_type behavior

Validation

• dbt compile -s customers --no-defer
• read-only warehouse previews checked row count, distinct customer_id, customer_type values, and spend arithmetic

Note: verification against dev with --defer has not been run yet.