Add PayloadID type and relation helpers, fix filterOptions casting issue

- Add PayloadID type for string | number IDs
- Add PayloadRelation<T> type for populated/unpopulated relations
- Add isPopulated() type guard to check if relation is populated
- Add resolveID() helper to extract ID from relation (object or ID)
- Add resolveIDs() helper for arrays of relations
- Fix filterOptions in Emails.ts to safely resolve ID before filtering
- This prevents MongoDB ObjectId casting errors when id is an object
- Bump version to 0.4.15

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

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

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@xtr-dev/payload-mailing",
-  "version": "0.4.14",
+  "version": "0.4.15",
   "description": "Template-based email system with scheduling and job processing for PayloadCMS",
   "type": "module",
   "main": "dist/index.js",