revert: completely remove all Sora functionality

backend/internal/handler/gateway_handler.go

@@ -859,14 +859,6 @@ func (h *GatewayHandler) Models(c *gin.Context) {
 		platform = forcedPlatform
 	}
 
-	if platform == service.PlatformSora {
-		c.JSON(http.StatusOK, gin.H{
-			"object": "list",
-			"data":   service.DefaultSoraModels(h.cfg),
-		})
-		return
-	}
-
 	// Get available models from account configurations (without platform filter)
 	availableModels := h.gatewayService.GetAvailableModels(c.Request.Context(), groupID, "")