fix: BigInt JSON serialization, pricing table dark mode

- Add BigInt.prototype.toJSON polyfill in main.ts so Express can
  serialize Prisma BigInt fields (priceVND, revenue amounts)
- Fix: admin/moderation and admin/revenue returning 500 Internal Error
- Fix pricing compare table: Enterprise column text invisible in dark
  mode (bg-green-50 without dark variant → add dark:bg-green-950/40)

Co-Authored-By: Claude Opus 4 (1M context) <noreply@anthropic.com>

apps/api/src/main.ts

@@ -1,5 +1,11 @@
 import './instrument';
 
+// BigInt cannot be serialized by JSON.stringify by default.
+// Polyfill toJSON so Express/NestJS can serialize Prisma BigInt fields.
+(BigInt.prototype as any).toJSON = function () {
+  return this.toString();
+};
+
 import { RequestMethod, ValidationPipe } from '@nestjs/common';
 import { NestFactory } from '@nestjs/core';
 import { DocumentBuilder, SwaggerModule } from '@nestjs/swagger';