feat: card popularity score

[hjacque]

Goal

Derive a composite popularity score per card (Product) by aggregating three sub-signals. This score feeds into the Opportunity Score (#91) as the demand-side weight — popular cards support stronger price floors and more reliable fair value ranges.

Sub-signals

Sub-signal	Issue	Role
Pokémon popularity score	#89	Inherent demand for the Pokémon(s) depicted
Multi-Pokémon card signal	#97	Bonus for cards depicting multiple Pokémon
Premium card symbol signal	#98	Bonus for exclusive symbols (Pokémon Center badge, event stamps, etc.)

Design notes

• Card-level score, computed from the three sub-signals above
• Stored on the Product (or a dedicated card metadata table)
• Recomputed when any sub-signal changes
• Exact combination weights require calibration from eBay sold data — start with an unweighted sum, tune empirically
• Feeds directly into popularitySignal column of OpportunityScore table (see feat: fair value range per card #91)

Blocked by

• feat: Pokémon popularity score #89 — Pokémon popularity score
• feat: multi-Pokémon card signal #97 — multi-Pokémon card signal
• feat: premium card symbol signal #98 — premium card symbol signal

Blocks

• feat: fair value range per card #91 — fair value range per card (Opportunity Score)