feat(marts): add fact_payments model (Phase 2.2)

[thoalecorn]

Related issue

Closses #6
##Description
Implements fact_payments mart model as specified in Phase 2.2.

Changes

• dbt/models/marts/facts/fact_payments.sql — built from stg_payments
  with order_purchase_date via LEFT JOIN with int_orders_enriched
• dbt/models/marts/facts/_fact_payments.yml — documents grain and
  defines not_null, unique, and relationships tests

Changes

• dbt/models/marts/facts/fact_payments.sql — built from stg_payments
  with order_purchase_date via LEFT JOIN with int_orders_enriched
• dbt/models/marts/facts/_fact_payments.yml — documents grain and
  defines not_null, unique, and relationships tests

Grain

1 row per payment_key (surrogate key of order_id + payment_sequential)

Join path & tradeoff

order_purchase_date is sourced from int_orders_enriched via LEFT JOIN
on order_id. int_orders_enriched was chosen over fact_orders to avoid
a mart-to-mart dependency, which is an anti-pattern in dbt layering.
LEFT JOIN ensures no payments are lost if an order_id has no match.

Verification

dbt build --project-dir ./dbt --profiles-dir ./dbt --select fact_payments
Done. PASS=9 WARN=0 ERROR=0 SKIP=0 TOTAL=9

[Flames4fun]

Great work!, merging right now.