fix(subscriptions): reject concurrent duplicate rules

Fixes #66

internal/handler/subscription_errors_test.go

@@ -0,0 +1,32 @@
+package handler
+
+import (
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/gin-gonic/gin"
+
+	"github.com/ShukeBta/MediaStationGo/internal/service"
+)
+
+func TestWriteSubscriptionConflictReturns409WithExistingID(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	w := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(w)
+	err := &service.SubscriptionAlreadyExistsError{ExistingID: "existing-sub"}
+	if !writeSubscriptionConflict(c, err) {
+		t.Fatal("expected conflict to be handled")
+	}
+	if w.Code != http.StatusConflict {
+		t.Fatalf("status = %d body=%s", w.Code, w.Body.String())
+	}
+	var body map[string]any
+	if err := json.Unmarshal(w.Body.Bytes(), &body); err != nil {
+		t.Fatal(err)
+	}
+	if body["existing_id"] != "existing-sub" {
+		t.Fatalf("body = %#v", body)
+	}
+}