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/dto/index.ts

@@ -0,0 +1,28 @@
+import { IsString, IsNotEmpty, Matches, IsOptional, IsBoolean } from "class-validator";
+
+export class CreateCronDto {
+  @IsString()
+  @IsNotEmpty()
+  expression: string;
+
+  @IsString()
+  @IsNotEmpty()
+  command: string;
+}
+
+export class UpdateCronDto {
+  @IsString()
+  @IsOptional()
+  @Matches(/^((\*|[0-9]|1[0-9]|2[0-9]|3[0-9]|4[0-9]|5[0-9])\ ?){5}$/, {
+    message: "Invalid cron expression",
+  })
+  expression?: string;
+
+  @IsString()
+  @IsOptional()
+  command?: string;
+
+  @IsBoolean()
+  @IsOptional()
+  enabled?: boolean;
+}