Support custom ID types (string/number) for improved compatibility

- Replace hardcoded payload-types imports with generic BaseEmailDocument interface
- Update sendEmail and sendEmailTask to work with both string and number IDs
- Refactor MailingService to use generic document types instead of specific ones
- Add BaseEmailDocument and BaseEmailTemplateDocument interfaces supporting id: string | number
- Export BaseEmailDocument for users to extend with their custom fields
- Fix TypeScript compilation error in template subject handling
- Add CUSTOM-TYPES.md documentation for users with different ID types

Fixes compatibility issue where plugin required number IDs but user projects used string IDs.

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

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

src/jobs/sendEmailTask.ts

@@ -1,5 +1,4 @@
-import { sendEmail } from '../sendEmail.js'
-import {Email, EmailTemplate} from '../payload-types.js'
+import { sendEmail, BaseEmailDocument } from '../sendEmail.js'
 
 export interface SendEmailTaskInput {
   // Template mode fields
@@ -170,7 +169,7 @@ export const sendEmailJob = {
       const sendEmailOptions = transformTaskInputToSendEmailOptions(taskInput)
 
       // Use the sendEmail helper to create the email
-      const email = await sendEmail<Email>(payload, sendEmailOptions)
+      const email = await sendEmail<BaseEmailDocument>(payload, sendEmailOptions)
 
       return {
         output: {