docs: add asynchronous jobs section to webapp README

Author: remote-swe-app[bot]

webapp/README.md

@@ -185,3 +185,68 @@ This project uses type-safe server actions with authentication:
      );
    }
    ```
+
+### Asynchronous Jobs
+
+Asynchronous jobs are Lambda functions that handle long-running or background tasks. The `job.Dockerfile` builds all TypeScript files in `src/jobs/` into separate Lambda handlers using `esbuild src/jobs/*.ts --bundle`.
+
+**Project structure:**
+
+For simple jobs, place a single file directly under `src/jobs/`:
+
+```
+webapp/src/jobs/
+├── migration-runner.ts           # Single-file Lambda handler
+└── async-job-runner.ts           # Single-file Lambda handler
+```
+
+For jobs with complex logic, use a subdirectory:
+
+```
+webapp/src/jobs/
+├── async-job-runner.ts           # Lambda handler entry point
+└── async-job/                    # Business logic directory
+    └── translate.ts              # Job implementation
+```
+
+**Example implementation:**
+
+```typescript
+// webapp/src/jobs/async-job-runner.ts
+import { translateJobHandler, translateJobSchema } from '@/jobs/async-job/translate';
+import { Handler } from 'aws-lambda';
+import { z } from 'zod';
+
+const jobPayloadPropsSchema = z.discriminatedUnion('type', [
+  translateJobSchema,
+  // Add more job types here
+]);
+
+export const handler: Handler<unknown> = async (event) => {
+  const { data: payload, error } = jobPayloadPropsSchema.safeParse(event);
+  if (error) throw new Error(error.toString());
+
+  switch (payload.type) {
+    case 'translate':
+      await translateJobHandler(payload);
+      break;
+  }
+};
+```
+
+```typescript
+// webapp/src/jobs/async-job/translate.ts
+import { z } from 'zod';
+
+export const translateJobSchema = z.object({
+  type: z.literal('translate'),
+  todoItemId: z.string(),
+  userId: z.string(),
+});
+
+export const translateJobHandler = async (params: z.infer<typeof translateJobSchema>) => {
+  // Job implementation
+};
+```
+
+**Note:** All jobs share the same `job.Dockerfile`. No individual Dockerfiles are needed. To deploy jobs, configure them in the CDK stack (see `cdk/lib/constructs/async-job.ts`).