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/index.ts

@@ -16,7 +16,7 @@ export { mailingJobs, sendEmailJob } from './jobs/index.js'
 export type { SendEmailTaskInput } from './jobs/sendEmailTask.js'
 
 // Main email sending function
-export { sendEmail, type SendEmailOptions } from './sendEmail.js'
+export { sendEmail, type SendEmailOptions, type BaseEmailDocument } from './sendEmail.js'
 export { default as sendEmailDefault } from './sendEmail.js'
 
 // Utility functions for developers