fix(backends): Backend-Löschen verweigern wenn Routing-Regeln darauf zeigen (v1.1.142)

- Service: Pre-Check auf routing_rules.backend_id → ErrInUse vor DELETE
- Handler: ErrInUse → HTTP 409 Conflict statt 500 (response.Conflict helper)
- UI: Delete-Button deaktiviert + Tooltip "Von N Regeln verwendet" wenn Referenzen existieren

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/handlers/backends.go

@@ -117,6 +117,10 @@ func (h *BackendsHandler) Delete(c *gin.Context) {
 			response.NotFound(c, err)
 			return
 		}
+		if errors.Is(err, backends.ErrInUse) {
+			response.Conflict(c, err)
+			return
+		}
 		response.Internal(c, err)
 		return
 	}

internal/handlers/response/response.go

@@ -82,6 +82,10 @@ func Forbidden(c *gin.Context, err error) {
 	Err(c, http.StatusForbidden, err)
 }
 
+func Conflict(c *gin.Context, err error) {
+	Err(c, http.StatusConflict, err)
+}
+
 func Internal(c *gin.Context, err error) {
 	Err(c, http.StatusInternalServerError, err)
 }

internal/services/backends/backends.go

@@ -14,7 +14,10 @@ import (
 	"git.netcell-it.de/projekte/edgeguard-native/internal/models"
 )
 
-var ErrNotFound = errors.New("backend not found")
+var (
+	ErrNotFound = errors.New("backend not found")
+	ErrInUse    = errors.New("backend is referenced by one or more routing rules")
+)
 
 type Repo struct {
 	Pool *pgxpool.Pool
@@ -99,6 +102,15 @@ RETURNING id, name, scheme, health_check_path, lb_algorithm, websocket, force_ht
 }
 
 func (r *Repo) Delete(ctx context.Context, id int64) error {
+	var n int
+	if err := r.Pool.QueryRow(ctx,
+		`SELECT COUNT(*) FROM routing_rules WHERE backend_id = $1`, id,
+	).Scan(&n); err != nil {
+		return err
+	}
+	if n > 0 {
+		return ErrInUse
+	}
 	tag, err := r.Pool.Exec(ctx, `DELETE FROM backends WHERE id = $1`, id)
 	if err != nil {
 		return err