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/migrations/20260719102746_add_shipping_rates_table/migration.sql

@@ -0,0 +1,16 @@
+-- CreateTable
+CREATE TABLE "ShippingRate" (
+    "id" TEXT NOT NULL,
+    "service" TEXT NOT NULL,
+    "basePrice" INTEGER NOT NULL,
+    "pricePerKg" INTEGER NOT NULL DEFAULT 0,
+    "estimatedDays" INTEGER NOT NULL,
+    "isInternational" BOOLEAN NOT NULL DEFAULT false,
+    "active" BOOLEAN NOT NULL DEFAULT true,
+    "updatedAt" TIMESTAMP(3) NOT NULL,
+
+    CONSTRAINT "ShippingRate_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateIndex
+CREATE UNIQUE INDEX "ShippingRate_service_key" ON "ShippingRate"("service");