Fix template population in beforeSend hook and bump version to 0.4.16

Added depth parameter to findByID call in processEmailItem to ensure template relationship is populated when passed to beforeSend hook.

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

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

src/services/MailingService.ts

@@ -233,6 +233,7 @@ export class MailingService implements IMailingService {
       const email = await this.payload.findByID({
         collection: this.emailsCollection as any,
         id: emailId,
+        depth: 1,
       }) as BaseEmailDocument
 
       // Combine from and fromName for nodemailer using proper sanitization