feat(workspace): replace supported platform count with bound media account count

The "platforms" stat tile was a constant (5) that never reflected what the
tenant actually had set up. Drop SupportedPlatformCount from the workspace
overview API/domain/shared-types and have admin-web compute a bound media
account count from the desktop accounts list, filtered through a new
isMediaPublishAccount helper that excludes deleted rows and AI-only
platforms (yuanbao/kimi/wenxin/deepseek/doubao/qwen).

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

server/internal/tenant/domain/workspace.go

@@ -6,7 +6,6 @@ type WorkspaceOverview struct {
 	ArticleCount           int64 `json:"article_count"`
 	PublishedCount         int64 `json:"published_count"`
 	BrandCount             int64 `json:"brand_count"`
-	SupportedPlatformCount int   `json:"supported_platform_count"`
 }
 
 type RecentArticle struct {