feat: Illustrator entity and card associations

[hjacque]

Goal

Track the illustrator of each card as a first-class entity. Artist identity is a meaningful part of a promo card's cultural weight and collectibility — Mitsuhiro Arita, Ken Sugimori, Atsuko Nishida each have their own collector following.

Design notes

• New Illustrator model: id, name, optionally nationality, activeYears
• Many-to-one relationship: Product → Illustrator (one card, one illustrator)
• Data entry likely manual initially; could later pull from Bulbapedia or TCGDex
• Enables: filter collection/wishlist by artist, "all Arita cards I'm missing", artist-based comps

Enables

• Illustrator as a dimension in popularity scoring (feat: Pokémon popularity score #89)
• Grouping and filtering in the dashboard by artist
• Artist portfolio view: all cards by a given illustrator, owned vs. wishlist