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/transport/router.go

@@ -149,6 +149,7 @@ func RegisterRoutes(app *bootstrap.App) {
 	articles.GET("", artHandler.List)
 	articles.POST("", artHandler.Create)
 	articles.POST("/generate-from-rule", artHandler.GenerateFromRule)
+	articles.POST("/imitations/generate", artHandler.GenerateImitation)
 	articles.POST("/:id/images", artHandler.UploadImage)
 	articles.POST("/:id/selection-optimize/stream", artHandler.OptimizeSelectionStream)
 	articles.GET("/:id", artHandler.Detail)