[Commerce] Add Etsy listing performance scoring from sold-order exports

[Nickel85]

Goal

Score Etsy listings using imported sold-order and order-item evidence so the store manager can identify winners, weak listings, and listings worth improving.

Signals

• Units sold.
• Revenue.
• Net order contribution.
• Coupon dependence.
• Average quantity per order.
• Physical vs digital fulfillment pattern.
• Variation demand, such as size or full-set selections.
• Recency of sales.

Acceptance Criteria

• Produces a listing performance summary per Listing ID and item name.
• Classifies listings into actionable buckets such as strong seller, promising, stagnant, discount-dependent, bundle candidate, and ad candidate.
• Explains the deterministic metrics behind each score.
• Does not require Etsy Ads data to produce baseline listing scores.
• Stores enough historical facts for future trend comparisons.
• Includes tests for listings with one sale, repeat sales, coupon-heavy sales, physical listings with shipping, and digital downloads.