fix(subscriptions): reject concurrent duplicate rules

Fixes #66

internal/handler/subscription_errors.go

@@ -0,0 +1,22 @@
+package handler
+
+import (
+	"errors"
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+
+	"github.com/ShukeBta/MediaStationGo/internal/service"
+)
+
+func writeSubscriptionConflict(c *gin.Context, err error) bool {
+	if !errors.Is(err, service.ErrSubscriptionAlreadyExists) {
+		return false
+	}
+	body := gin.H{"error": "subscription already exists"}
+	if existingID := service.SubscriptionAlreadyExistsID(err); existingID != "" {
+		body["existing_id"] = existingID
+	}
+	c.JSON(http.StatusConflict, body)
+	return true
+}