Add products, inventory units, stock receipts, and price history

- product table (catalog definition, no cost column — cost tracked
  per receipt/unit)
- inventory_unit table (serialized items with serial number,
  condition, status)
- stock_receipt table (FIFO cost tracking — records every stock
  receive event with cost_per_unit, supplier, date)
- price_history table (logs every retail price change for margin
  analysis over time)
- product_supplier join table (many-to-many, tracks supplier SKU
  and preferred supplier)
- Full CRUD routes + search (name, SKU, UPC, brand)
- Inventory unit routes nested under products
- Price changes auto-logged on product update
- 33 tests passing

packages/backend/src/db/migrations/meta/_journal.json

@@ -36,6 +36,13 @@
       "when": 1774652800605,
       "tag": "0004_peaceful_wendell_rand",
       "breakpoints": true
+    },
+    {
+      "idx": 5,
+      "version": "7",
+      "when": 1774653515690,
+      "tag": "0005_add_products_units_receipts",
+      "breakpoints": true
     }
   ]
 }