fix: keep Polymarket per-market image in UnifiedMarket mapping

[saschabuehrle]

Bug

Fixes #62 — multi-market Polymarket events were assigning the same event-level image to every UnifiedMarket, even when the underlying market had its own image.

Fix

• Updated Polymarket market normalization image precedence to prefer market-level image first.
• New precedence order:
  1. market.image
  2. event.image
  3. fallback OG image URL
• Added a focused unit test that verifies market image wins when both event and market images are present.

Files changed

• core/src/exchanges/polymarket/utils.ts
• core/test/unit/normalizers/polymarket.test.ts

Testing

• Added unit test coverage for the image precedence behavior in the Polymarket normalizer test suite.

Happy to adjust if you want stricter fallback behavior for empty-string image values as well.

Greetings, saschabuehrle

[realfishsam]

Good stuff!