Refactor product extraction to use configuration-driven approach

[vertefra]

Problem

extractAmazonProduct() and extractWalmartProduct() in utils/productExtractor.ts have identical structure but different selectors. This makes:

• Adding new marketplaces require duplicating ~50 lines of code
• Selector updates need changes in multiple places
• Hard to maintain and test

Solution

Use a configuration-driven approach with marketplace configs and generic extraction functions.

See full implementation in the original review document.

Benefits

• Eliminates ~100 lines of duplicate code
• Adding new marketplace = just add config object
• Easier to test and maintain
• Centralized selector management

Priority

High