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/components/PublishArticleModal.vue

@@ -21,9 +21,6 @@ import {
   publishStateLabel,
   type PublishAccountCard,
 } from "@/lib/publish-account-cards";
-import {
-  normalizePublishPlatformIds,
-} from "@/lib/publish-platforms";
 
 const props = defineProps<{
   open: boolean;
@@ -93,10 +90,8 @@ watch(
 
 const platformMap = computed(() => buildPublishPlatformMap(platformsQuery.data.value ?? []));
 
-const articlePlatformIds = computed(() => normalizePublishPlatformIds(detailQuery.data.value?.platforms ?? []));
-
 const accountCards = computed<PublishAccountCard[]>(() =>
-  buildPublishAccountCards(accountsQuery.data.value ?? [], platformMap.value, articlePlatformIds.value),
+  buildPublishAccountCards(accountsQuery.data.value ?? [], platformMap.value),
 );
 
 watch(
@@ -137,10 +132,7 @@ watchEffect(() => {
     return;
   }
 
-  const preferredPlatforms = new Set(articlePlatformIds.value);
-  selectedAccountIds.value = accountCards.value
-    .filter((card) => card.selectable && preferredPlatforms.has(card.platformId))
-    .map((card) => card.id);
+  selectedAccountIds.value = [];
   selectionHydrated.value = true;
 });
 
@@ -237,7 +229,6 @@ async function persistCoverIfNeeded(detail: ArticleDetail): Promise<ArticleDetai
   return articlesApi.update(detail.id, {
     title: detail.title?.trim() || t("article.untitled"),
     markdown_content: detail.markdown_content ?? "",
-    platforms: normalizePublishPlatformIds(detail.platforms ?? []),
     cover_asset_url: nextUrl || null,
     cover_image_asset_id: coverImageAssetId.value,
   });