feat(landing,billing): public pricing, global discount, services bar & estimator

Backend
- Add platform-wide global discount (platform_settings: global_discount_percent),
  applied centrally in PricingCatalogService.computeTotalsFromDb so it reaches
  every real charge (previews, deploys, renewals, upgrades, invoices). Admin
  GET/PATCH /billing/settings/global-discount.
- Add unauthenticated PublicPricingController (catalog + calculate) for the
  public landing page, returning gross/net and the discount percentage.
- Bill application replicas by the user-selected footprint: app CPU/RAM/storage
  now all scale by replica count; the single-replica database stays unscaled.

Frontend
- Landing: Services bar (PaaS active, DBaaS, KaaS/LaaS "coming soon" with
  expandable runtime/database menus), transparent Pricing section (per-resource
  rate cards with cycle toggle + discount strikethrough), and a cost Estimator
  ("estimate your package").
- Optional services and the database are priced like runtimes: the estimator
  lets users pick their CPU/RAM/storage (and DB type) so the cost scales by need.
- Admin billing: global-discount editor.
- i18n: fa/en strings for services, pricing, estimator and global discount.

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

backend/src/billing/billing.service.ts

@@ -48,6 +48,18 @@ export class BillingService {
     return this.pricingCatalog.updateCatalog(dto);
   }
 
+  // ─── Global (platform-wide) discount ──────────────────────────────
+
+  /** Current platform-wide discount percentage (0–100). */
+  async getGlobalDiscount(): Promise<{ percentOff: number }> {
+    return { percentOff: await this.pricingCatalog.getGlobalDiscountPercent(true) };
+  }
+
+  /** Set the platform-wide discount percentage (Admin). */
+  async setGlobalDiscount(percentOff: number): Promise<{ percentOff: number }> {
+    return { percentOff: await this.pricingCatalog.setGlobalDiscountPercent(percentOff) };
+  }
+
   // ─── Cost Calculation ─────────────────────────────────────────────
 
   /**