feat(publish): require cover image for WeChat Official Account

WeChat 公众号在管理员/运营者授权发布场景下需要 2.35:1 封面，
新增共享 publishPlatformRequiresCover 校验并同步前端封面要求与文案，
避免在调度与即时发布两条路径上重复维护平台名单。

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

server/internal/tenant/app/publish_platforms_test.go

@@ -0,0 +1,30 @@
+package app
+
+import "testing"
+
+func TestPublishPlatformRequiresCover(t *testing.T) {
+	t.Parallel()
+
+	for _, platformID := range []string{"weixin_gzh", "baijiahao", "dongchedi"} {
+		if !publishPlatformRequiresCover(platformID) {
+			t.Fatalf("publishPlatformRequiresCover(%q) = false, want true", platformID)
+		}
+	}
+
+	if publishPlatformRequiresCover("zhihu") {
+		t.Fatal("publishPlatformRequiresCover(\"zhihu\") = true, want false")
+	}
+}
+
+func TestPublishCoverRequirementCallersIncludeWeixinGzh(t *testing.T) {
+	t.Parallel()
+
+	accounts := []platformAccountSeed{{PlatformID: "weixin_gzh"}}
+	if !publishBatchRequiresCover(accounts) {
+		t.Fatal("publishBatchRequiresCover() = false for weixin_gzh, want true")
+	}
+
+	if !schedulePublishPlatformsRequireCover([]string{"weixin_gzh"}) {
+		t.Fatal("schedulePublishPlatformsRequireCover() = false for weixin_gzh, want true")
+	}
+}