Refactor and clean up job organization

- Properly encapsulate processEmailsJob in its own file with handler and definition
- Clean up index.ts to remove duplicate code and just export job definitions
- Add comprehensive JSDoc comments for better documentation
- Separate job handler logic from job definition for clarity
- Fix job scheduling to use correct field names
- Bump version to 0.2.1

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@xtr-dev/payload-mailing",
-  "version": "0.2.0",
+  "version": "0.2.1",
   "description": "Template-based email system with scheduling and job processing for PayloadCMS",
   "type": "module",
   "main": "dist/index.js",