feat(metrics): invalidate the user count cache on registration

Registration is the one hot path where instant freshness is worth an
extra COUNT(*), so bust the cache there rather than waiting for the TTL.

pkg/routes/api/v1/user_register.go

@@ -22,6 +22,8 @@ import (
 
 	"code.vikunja.io/api/pkg/config"
 	"code.vikunja.io/api/pkg/db"
+	"code.vikunja.io/api/pkg/log"
+	"code.vikunja.io/api/pkg/metrics"
 	"code.vikunja.io/api/pkg/models"
 	"code.vikunja.io/api/pkg/user"
 
@@ -85,5 +87,13 @@ func RegisterUser(c *echo.Context) error {
 		return err
 	}
 
+	// Bust the cached user count so the new registration shows up in metrics
+	// immediately instead of after the regular cache expiry.
+	if config.MetricsEnabled.GetBool() {
+		if err := metrics.InvalidateCount(metrics.UserCountKey); err != nil {
+			log.Errorf("Could not invalidate user count metric: %s", err)
+		}
+	}
+
 	return c.JSON(http.StatusOK, newUser)
 }