Add sendEmail workflow with immediate processing option

- Create sendEmailWorkflow as a Payload workflow alternative to task
- Add processImmediately option (disabled by default) to send emails immediately
- Expose processEmailItem method in MailingService for individual email processing
- Add comprehensive input schema with conditional fields
- Update plugin to register both tasks and workflows
- Add detailed documentation comparing tasks vs workflows
- Includes status tracking and error handling
- Bump version to 0.3.0 (new feature)

src/services/MailingService.ts

@@ -225,7 +225,7 @@ export class MailingService implements IMailingService {
     }
   }
 
-  private async processEmailItem(emailId: string): Promise<void> {
+  async processEmailItem(emailId: string): Promise<void> {
     try {
       await this.payload.update({
         collection: this.emailsCollection as any,