feat: premium card symbol signal

[hjacque]

Goal

Surface a premium demand signal for cards carrying exclusive symbols or markings — indicators of provenance, rarity tier, or event exclusivity that drive collector premiums beyond what rarity alone explains.

Examples of premium symbols

• Pokémon Center badge — exclusive to Pokémon Center releases
• Event stamps — tournament, championship, or promotional event markings
• Regional exclusives — symbols tied to specific country or retailer exclusives (Japan, Korea, Taiwan/HK)

Signal

• Boolean or tiered flag per card (e.g. no symbol / standard promo / premium symbol)
• Presence of a premium symbol amplifies 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)
• Symbols need to be catalogued per Product — initially may require manual tagging

Blocked by

• None (can be data-modelled independently)