Feature: Add database-driven shipping rates management

- Add ShippingRate model to Prisma schema with service name, base price, per-kg surcharge, estimated delivery days, and international flag
- Create migration to add shipping_rates table
- Update seed.ts to populate initial Royal Mail rates (domestic and international)
- Refactor getRoyalMailShippingQuotes to fetch rates from database with fallback to defaults
- Add /admin/shipping-rates page for managing rates
- Add ShippingRatesForm component with inline editing and add/remove functionality
- Add server actions for saving and deleting rates
- Add "Shipping rates" link to admin navigation

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

prisma/seed.ts

@@ -123,6 +123,44 @@ const products: Array<{
   },
 ];
 
+const shippingRates = [
+  {
+    service: 'Special Delivery Guaranteed by 9am',
+    basePrice: 895,
+    pricePerKg: 0,
+    estimatedDays: 1,
+    isInternational: false,
+  },
+  {
+    service: 'Royal Mail 24®',
+    basePrice: 495,
+    pricePerKg: 0,
+    estimatedDays: 1,
+    isInternational: false,
+  },
+  {
+    service: 'Royal Mail 48®',
+    basePrice: 295,
+    pricePerKg: 0,
+    estimatedDays: 3,
+    isInternational: false,
+  },
+  {
+    service: 'International Signed',
+    basePrice: 1495,
+    pricePerKg: 300,
+    estimatedDays: 14,
+    isInternational: true,
+  },
+  {
+    service: 'International Economy',
+    basePrice: 1095,
+    pricePerKg: 200,
+    estimatedDays: 21,
+    isInternational: true,
+  },
+];
+
 async function main() {
   for (const c of categories) {
     await prisma.category.upsert({
@@ -153,7 +191,17 @@ async function main() {
     });
   }
 
-  console.log(`Seeded ${categories.length} categories and ${products.length} products.`);
+  for (const rate of shippingRates) {
+    await prisma.shippingRate.upsert({
+      where: { service: rate.service },
+      update: { ...rate },
+      create: { ...rate },
+    });
+  }
+
+  console.log(
+    `Seeded ${categories.length} categories, ${products.length} products, and ${shippingRates.length} shipping rates.`
+  );
 }
 
 main()