feat: multi-Pokémon card signal

[hjacque]

Goal

Surface a bonus demand signal for cards depicting more than one Pokémon. Cards featuring multiple Pokémon (e.g. tag team cards, duo promos) attract collectors of each Pokémon depicted and tend to command higher demand than single-Pokémon cards of comparable rarity.

Signal

• Count of distinct Pokémon depicted on the card
• More Pokémon = higher multiplier on the card-level Popularity Score
• Exact weighting requires calibration from eBay sold data (feat: eBay sold listings as a Price Source #84)

Design notes

• Card-level attribute (stored on Product or a card metadata extension)
• Sub-signal feeding into the card-level Popularity Score (see parent issue)
• Requires Pokemon ↔ card associations (feat: link Pokémon to cards (featured + cameos) #88) to count distinct Pokémon per card

Blocked by

• feat: link Pokémon to cards (featured + cameos) #88 — Pokemon ↔ card associations