feat(#29): implement cron job configuration

- Add CronSchedule model to Prisma schema
- Implement CronService with CRUD operations
- Add REST API endpoints for cron management
- Create MoltBot plugin skill definition (SKILL.md)
- TDD: 9 passing tests for CronService

apps/api/src/cron/cron.module.ts

@@ -0,0 +1,13 @@
+import { Module } from "@nestjs/common";
+import { CronController } from "./cron.controller";
+import { CronService } from "./cron.service";
+import { PrismaModule } from "../prisma/prisma.module";
+import { AuthModule } from "../auth/auth.module";
+
+@Module({
+  imports: [PrismaModule, AuthModule],
+  controllers: [CronController],
+  providers: [CronService],
+  exports: [CronService],
+})
+export class CronModule {}