feat(billing): add percentage discount coupons

Admins can create coupon codes that discount specific services (app
runtimes, optional services, managed products, custom-domain addon, or
all) and restrict them to specific users or make them public, with total
and per-user usage caps and an active date window.

Coupons apply in deploy, renewal, and upgrade flows: cost-breakdown lines
are tagged with a service key, the eligible portion is discounted and
capped to the payable amount, the invoice records discountAmount/
discountCode, and the redemption is recorded once when the invoice is
fully paid (covering wallet, gateway, and mixed payments).

- Discount + DiscountRedemption entities; invoice discount columns
- DiscountService (CRUD, validation, redemption) + admin/validate API
- Idempotent schema bootstrap on init so production (synchronize off)
  provisions the tables/columns without a migration runner
- Admin discounts UI, coupon entry in deploy/renewal, invoice discount line
- fa/en strings; discount.service unit spec

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/i18n/dictionaries/en.ts

@@ -655,6 +655,7 @@ const en: Dictionary = {
       },
       title: 'Invoices',
       subtitle: 'Review what each payment was for and pay open invoices.',
+      subtotal: 'Subtotal',
       walletBalance: 'Wallet balance',
       toman: 'Toman',
       filterAll: 'All',
@@ -1102,6 +1103,69 @@ const en: Dictionary = {
       saveFailedShort: 'Failed to save',
       hours: 'hours',
       days: 'days',
+      discounts: {
+        title: 'Discount codes',
+        subtitle: 'Percentage discounts on different services — public or for specific users.',
+        add: 'New discount',
+        empty: 'No discount codes yet',
+        edit: 'Edit',
+        delete: 'Delete',
+        deleteConfirm: 'Delete this discount code?',
+        code: 'Code',
+        codePlaceholder: 'NOWRUZ1403',
+        name: 'Label',
+        namePlaceholder: 'Nowruz discount',
+        description: 'Description',
+        percentOff: 'Percent off',
+        scope: 'Eligible services',
+        allServices: 'All services',
+        selectServices: 'Select specific services',
+        audience: 'Eligible users',
+        public: 'Public (all users)',
+        restricted: 'Specific users',
+        searchUsers: 'Search users by name or email…',
+        noUsersSelected: 'No users selected yet',
+        limits: 'Limits',
+        maxUses: 'Total usage cap',
+        maxUsesPerUser: 'Per-user cap',
+        unlimited: 'Unlimited',
+        startsAt: 'Start date',
+        endsAt: 'End date',
+        active: 'Active',
+        inactive: 'Inactive',
+        used: 'Used',
+        save: 'Save',
+        saving: 'Saving…',
+        cancel: 'Cancel',
+        saved: 'Discount code saved',
+        deleted: 'Discount code deleted',
+        saveFailed: 'Failed to save discount code',
+        groups: {
+          runtime: 'App runtimes',
+          optional: 'Optional services',
+          managed: 'Managed services',
+          addon: 'Add-ons',
+        },
+        coupon: {
+          label: 'Discount code',
+          placeholder: 'Enter discount code',
+          apply: 'Apply',
+          checking: 'Checking…',
+          applied: 'Discount applied',
+          remove: 'Remove',
+          discountLine: 'Discount',
+        },
+        reasons: {
+          not_found: 'Invalid discount code',
+          inactive: 'This discount code is inactive',
+          not_started: 'This code is not active yet',
+          expired: 'This code has expired',
+          max_uses_reached: 'This code has reached its usage limit',
+          max_uses_per_user_reached: 'You have reached your usage limit for this code',
+          not_eligible_user: 'This code is not available for your account',
+          no_eligible_services: 'This code does not discount your selected services',
+        },
+      },
     },
     servicesNew: {
       steps: ['Service type', 'Configuration', 'Review & pay'],