feat: add custom domain support with SSL, DNS verification, and billing

Users can assign a custom domain to their app with automatic SSL via
cert-manager. Includes DNS verification flow (CNAME check), Persian
instructions, admin-configurable pricing via PlatformSetting, and
integration into the deploy wizard cost calculation.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/src/billing/billing.controller.ts

@@ -95,6 +95,24 @@ export class BillingController {
     return this.billingService.calculateCost(dto);
   }
 
+  // ─── Custom Domain Pricing ─────────────────────────────────────
+
+  @Get('settings/custom-domain-price')
+  @ApiOperation({ summary: 'Get custom domain monthly price' })
+  async getCustomDomainPrice() {
+    return this.billingService.getCustomDomainPrice();
+  }
+
+  @Patch('settings/custom-domain-price')
+  @Roles(UserRole.ADMIN)
+  @ApiOperation({ summary: 'Set custom domain monthly price (Admin)' })
+  async setCustomDomainPrice(@Body() body: { monthlyPrice: number }) {
+    if (body.monthlyPrice === undefined || body.monthlyPrice < 0) {
+      throw new BadRequestException('monthlyPrice must be a non-negative number');
+    }
+    return this.billingService.setCustomDomainPrice(body.monthlyPrice);
+  }
+
   // ─── Wallet (User) ───────────────────────────────────────────────
 
   @Get('wallet')