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>

src/types/index.ts

@@ -1,6 +1,12 @@
 import { Payload } from 'payload'
 import type { CollectionConfig, RichTextField } from 'payload'
 
+// Payload ID type (string or number)
+export type PayloadID = string | number
+
+// Payload relation type - can be populated (object with id) or unpopulated (just the ID)
+export type PayloadRelation<T extends { id: PayloadID }> = T | PayloadID
+
 // JSON value type that matches Payload's JSON field type
 export type JSONValue = string | number | boolean | { [k: string]: unknown } | unknown[] | null | undefined