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)
 }