feat(article): expose auto_publish_platforms derived from publish accounts

Replace the freeform `platforms` field on articles/tasks (parsed out of
wizard_state/input_params JSON) with `auto_publish_platforms`, resolved
by joining schedule publish accounts to platform_accounts. The migration
strips the legacy keys from existing rows so the new field is the single
source of truth.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/admin-web/src/i18n/messages/en-US.ts

@@ -1290,9 +1290,9 @@ const enUS = {
       name: "Task name",
       rule: "Prompt rule",
       cron: "Frequency",
+      autoPublishPlatforms: "Auto publish platforms",
       autoPublish: "Auto publish",
       manualPublish: "Manual publish",
-      autoPublishAccountCount: "Auto · {count} accounts",
       nextRun: "Next run",
       startAt: "Start time",
       endAt: "End time",