🚀 BREAKING: Simplify API to use Payload collections directly

- Remove complex sendEmail/scheduleEmail methods and SendEmailOptions types
- Add simple renderTemplate() helper for template rendering
- Users now create emails using payload.create() with full type safety
- Leverage Payload's existing collection system instead of duplicating functionality
- Provide comprehensive migration guide and usage examples

BREAKING CHANGES:
- sendEmail() and scheduleEmail() methods removed
- SendEmailOptions type removed
- Use payload.create() with email collection instead
- Use renderTemplate() helper for template rendering

Benefits:
✅ Full TypeScript support with generated Payload types
✅ Use any custom fields in your email collection
✅ Leverage Payload's validation, hooks, and access control
✅ Simpler, more consistent API
✅ Less code to maintain

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/utils/helpers.ts

@@ -1,5 +1,5 @@
 import { Payload } from 'payload'
-import { SendEmailOptions } from '../types/index.js'
+import { TemplateVariables } from '../types/index.js'
 
 export const getMailing = (payload: Payload) => {
   const mailing = (payload as any).mailing
@@ -9,14 +9,9 @@ export const getMailing = (payload: Payload) => {
   return mailing
 }
 
-export const sendEmail = async (payload: Payload, options: SendEmailOptions): Promise<string> => {
+export const renderTemplate = async (payload: Payload, templateSlug: string, variables: TemplateVariables): Promise<{ html: string; text: string; subject: string }> => {
   const mailing = getMailing(payload)
-  return mailing.service.sendEmail(options)
-}
-
-export const scheduleEmail = async (payload: Payload, options: SendEmailOptions): Promise<string> => {
-  const mailing = getMailing(payload)
-  return mailing.service.scheduleEmail(options)
+  return mailing.service.renderTemplate(templateSlug, variables)
 }
 
 export const processEmails = async (payload: Payload): Promise<void> => {