feat: Pokémon popularity score

[hjacque]

Goal

Derive a popularity score per Pokémon entity — the inherent demand signal for a given Pokémon independent of any specific card. Popular Pokémon (Charizard, Pikachu, Gengar) command stronger price floors and more resilient demand across all cards featuring them.

Potential signals to explore

• PSA grading volume — total pop across all cards featuring this Pokémon; high grading activity = high collector demand
• Price premium — do cards featuring this Pokémon trade at a premium vs. comparable cards of similar rarity/era?
• eBay sold listing frequency — transaction volume for this Pokémon (once feat: eBay sold listings as a Price Source #84 is in place)
• External popularity data — Pokémon GO usage stats, competitive play frequency, fan rankings (research: external Pokémon popularity data sources #101)

Design notes

• Score lives on the Pokemon entity, recomputed periodically
• Sub-signal feeding into the card-level Popularity Score (feat: card popularity score #99)
• Not a card-level signal on its own — card-specific factors (symbols, number of Pokémon depicted) are handled in sibling issues (feat: multi-Pokémon card signal #97, feat: premium card symbol signal #98)

Blocked by

• feat: Pokemon entity — seed all Pokémon in database #87 — Pokémon entity
• feat: link Pokémon to cards (featured + cameos) #88 — Pokémon ↔ card associations
• research: external Pokémon popularity data sources #101 — research: external Pokémon popularity data sources

Blocks

• feat: card popularity score #99 — card popularity score