feat: add product sold event handling

Add sell_product() service method that creates ProductSold events
with calculated unit_price_cents (floor division of total/qty).
Update ProductsProjection to handle PRODUCT_SOLD events.

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

PLAN.md

@@ -179,11 +179,11 @@ Check off items as completed. Each phase builds on the previous.
 - [x] **Commit checkpoint** (c08fa47)
 
 ### Step 4.5: Product Sold Event
-- [ ] Implement apply_product_sold projection
-- [ ] Add sell_product to services/products.py
-- [ ] Calculate unit_price_cents = floor(total/qty)
-- [ ] Write tests: event stored, unit price calculated
-- [ ] **Commit checkpoint**
+- [x] Implement apply_product_sold projection
+- [x] Add sell_product to services/products.py
+- [x] Calculate unit_price_cents = floor(total/qty)
+- [x] Write tests: event stored, unit price calculated
+- [x] **Commit checkpoint**
 
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