feat(imitation): add article imitation create flow

Introduce 仿写创作: new list and create views, backend imitation service
and prompt templates, worker task routing for imitation jobs, and one-click
rewrite triggers from question citation sources. Surface source article
URL/title on article list/detail, and restrict failed articles to delete-only.

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

server/internal/tenant/app/template_service.go

@@ -241,7 +241,7 @@ func (s *TemplateService) SaveDraft(ctx context.Context, templateID int64, req S
 		  AND template_id = $4
 		  AND source_type = 'template'
 		  AND deleted_at IS NULL
-		  AND generate_status IN ('draft', 'failed')
+		  AND generate_status = 'draft'
 		RETURNING id
 	`, stateJSON, *req.ArticleID, actor.TenantID, templateID).Scan(&articleID)
 	if err != nil {
@@ -333,7 +333,7 @@ func (s *TemplateService) Generate(ctx context.Context, templateID int64, req Ge
 			  AND template_id = $3
 			  AND source_type = 'template'
 			  AND deleted_at IS NULL
-			  AND generate_status IN ('draft', 'failed')
+			  AND generate_status = 'draft'
 			FOR UPDATE
 		`, *req.ArticleID, actor.TenantID, templateID).Scan(&articleID)
 		if err != nil {
@@ -630,6 +630,8 @@ func generationFailureStageLabel(stage string) string {
 		return "AI 正文生成失败"
 	case "knowledge_resolve":
 		return "知识库检索失败"
+	case "source_fetch":
+		return "源文章抓取失败"
 	case "persist_begin_tx":
 		return "写入生成结果事务启动失败"
 	case "persist_create_version":