feat: implement order workflow, metrics, and handler unit tests

- implement UpdateOrderStatus and GetOrdersReport service methods
- implement all order, user handlers (UpdateMe, GetMyTrips, CancelOrder, GetOrder, etc.)
- extract OrderServicer interface for testability
- add Prometheus metrics middleware (requests total, duration)
- fix GetAll storage flavor for PostgreSQL ($1 placeholders)
- add 17 unit tests for order handlers via httptest
- wire Grafana datasource and update Prometheus scrape config
- update README with full API reference, pricing, roles, monitoring

internal/handler/manager.go

@@ -70,6 +70,7 @@ func (s *Server) CreateManager(w http.ResponseWriter, r *http.Request) {
 	if err := storage.Create(ctx, "users", userModel, tx); err != nil {
 		slog.ErrorContext(ctx, "Error while creating user", slog.String("error", err.Error()))
 		s.JSON(w, r, http.StatusInternalServerError, MsgInternalError, RespError)
+		tx.Rollback(ctx)
 		return
 	}
 
@@ -83,6 +84,7 @@ func (s *Server) CreateManager(w http.ResponseWriter, r *http.Request) {
 	if err := storage.Create(ctx, "managers", managerModel, tx); err != nil {
 		slog.ErrorContext(ctx, "Error while creating manager", slog.String("error", err.Error()))
 		s.JSON(w, r, http.StatusInternalServerError, MsgInternalError, RespError)
+		tx.Rollback(ctx)
 		return
 	}