Fix scheduledAt type in SendEmailTaskInput and add Date normalization

- Update SendEmailTaskInput.scheduledAt to support string | Date types
- Add Date object normalization to ISO strings in sendEmail processing
- Ensure consistent database storage format for all timestamp fields
- Convert Date objects to ISO strings before database operations

Resolves remaining "Type Date is not assignable to type string" error
for scheduledAt field in job task input.

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

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

src/sendEmail.ts

@@ -100,6 +100,23 @@ export const sendEmail = async <TEmail extends BaseEmailDocument = BaseEmailDocu
     emailData.from = validated && validated.length > 0 ? validated[0] : undefined
   }
 
+  // Normalize Date objects to ISO strings for consistent database storage
+  if (emailData.scheduledAt instanceof Date) {
+    emailData.scheduledAt = emailData.scheduledAt.toISOString()
+  }
+  if (emailData.sentAt instanceof Date) {
+    emailData.sentAt = emailData.sentAt.toISOString()
+  }
+  if (emailData.lastAttemptAt instanceof Date) {
+    emailData.lastAttemptAt = emailData.lastAttemptAt.toISOString()
+  }
+  if (emailData.createdAt instanceof Date) {
+    emailData.createdAt = emailData.createdAt.toISOString()
+  }
+  if (emailData.updatedAt instanceof Date) {
+    emailData.updatedAt = emailData.updatedAt.toISOString()
+  }
+
   // Create the email in the collection with proper typing
   const email = await payload.create({
     collection: collectionSlug,