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/schema.prisma

@@ -546,3 +546,17 @@ model BankReconciliation {
   confidence          Int      @default(100) // 0-100, how confident the match is (100=exact amount+date, <100=fuzzy match)
   notes               String?  // manual notes about the match
 }
+
+// Shipping rates configuration — managed from admin panel
+// Allows easy updates to Royal Mail pricing without code changes
+model ShippingRate {
+  id                String   @id @default(cuid())
+  service           String   @unique // e.g., "Royal Mail 24®", "International Signed"
+  basePrice         Int      // cents — base price for this service
+  pricePerKg        Int      @default(0) // cents — additional cost per kg (mainly for international)
+  estimatedDays     Int      // estimated delivery days
+  isInternational   Boolean  @default(false)
+  active            Boolean  @default(true)
+  updatedAt         DateTime @updatedAt
+}
+