fix: ensure user source is created in the sharePost mutation

[omBratteng]

Missed doing the "polyfill" in the sharePost mutation.

https://dailydotdev.slack.com/archives/C01L0QXQJNB/p1754424084071199

[pulumi]

🍹 The Update (preview) for dailydotdev/api/prod (at 9e61dac) was successful.

Resource Changes

    Name                                            Type                           Operation
~   vpc-native-update-source-tag-view-cron          kubernetes:batch/v1:CronJob    update
+   vpc-native-api-migration-49201871               kubernetes:batch/v1:Job        create
~   vpc-native-update-tags-str-cron                 kubernetes:batch/v1:CronJob    update
~   vpc-native-clean-zombie-users-cron              kubernetes:batch/v1:CronJob    update
~   vpc-native-generic-referral-reminder-cron       kubernetes:batch/v1:CronJob    update
~   vpc-native-validate-active-users-cron           kubernetes:batch/v1:CronJob    update
-   vpc-native-api-migration-627807ab               kubernetes:batch/v1:Job        delete
~   vpc-native-clean-zombie-user-companies-cron     kubernetes:batch/v1:CronJob    update
~   vpc-native-personalized-digest-deployment       kubernetes:apps/v1:Deployment  update
~   vpc-native-hourly-notification-cron             kubernetes:batch/v1:CronJob    update
~   vpc-native-personalized-digest-cron             kubernetes:batch/v1:CronJob    update
~   vpc-native-update-current-streak-cron           kubernetes:batch/v1:CronJob    update
~   vpc-native-update-source-public-threshold-cron  kubernetes:batch/v1:CronJob    update
~   vpc-native-clean-gifted-plus-cron               kubernetes:batch/v1:CronJob    update
~   vpc-native-calculate-top-readers-cron           kubernetes:batch/v1:CronJob    update
~   vpc-native-clean-stale-user-transactions-cron   kubernetes:batch/v1:CronJob    update
~   vpc-native-private-deployment                   kubernetes:apps/v1:Deployment  update
~   vpc-native-bg-deployment                        kubernetes:apps/v1:Deployment  update
~   vpc-native-deployment                           kubernetes:apps/v1:Deployment  update
~   vpc-native-daily-digest-cron                    kubernetes:batch/v1:CronJob    update
~   vpc-native-temporal-deployment                  kubernetes:apps/v1:Deployment  update
~   vpc-native-update-trending-cron                 kubernetes:batch/v1:CronJob    update
~   vpc-native-sync-subscription-with-cio-cron      kubernetes:batch/v1:CronJob    update
~   vpc-native-clean-zombie-images-cron             kubernetes:batch/v1:CronJob    update
~   vpc-native-generate-search-invites-cron         kubernetes:batch/v1:CronJob    update
~   vpc-native-update-views-cron                    kubernetes:batch/v1:CronJob    update
~   vpc-native-update-tag-recommendations-cron      kubernetes:batch/v1:CronJob    update
~   vpc-native-update-highlighted-views-cron        kubernetes:batch/v1:CronJob    update
~   vpc-native-ws-deployment                        kubernetes:apps/v1:Deployment  update
~   vpc-native-check-analytics-report-cron          kubernetes:batch/v1:CronJob    update