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/main.ts

@@ -9,6 +9,7 @@ import { healthRoutes } from './routes/v1/health.js'
 import { authRoutes } from './routes/v1/auth.js'
 import { accountRoutes } from './routes/v1/accounts.js'
 import { inventoryRoutes } from './routes/v1/inventory.js'
+import { productRoutes } from './routes/v1/products.js'
 
 export async function buildApp() {
   const app = Fastify({
@@ -37,6 +38,7 @@ export async function buildApp() {
   await app.register(authRoutes, { prefix: '/v1' })
   await app.register(accountRoutes, { prefix: '/v1' })
   await app.register(inventoryRoutes, { prefix: '/v1' })
+  await app.register(productRoutes, { prefix: '/v1' })
 
   return app
 }