Per-product commission rates in the affiliate program

**Problem:** Currently, the affiliate program applies a single commission rate across all products. This limits flexibility for merchants who want to incentivize affiliates differently based on product margins, pricing tiers, or strategic priorities. **Proposed Solution:** Allow merchants to configure affiliate commissions at the product level: - **Product selection:** Choose which specific products are included in the affiliate program (rather than all-or-nothing) - **Per-product commission rates:** Set a custom commission percentage or fixed amount for each selected product - **Default fallback:** Keep a default commission rate for products without a specific override **Use Cases:** - Higher commission on high-margin products to drive affiliate focus - Lower commission on already-popular products that sell themselves - Exclude certain products (e.g., loss leaders, bundles) from affiliate payouts - Run time-limited commission boosts on specific products **Expected Outcome:** Merchants get granular control over their affiliate economics, leading to better-optimized affiliate programs and more strategic product promotion.