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

configs/datasources/prometheus.yaml

@@ -0,0 +1,7 @@
+apiVersion: 1
+datasources:
+  - name: Prometheus
+    type: prometheus
+    url: http://logiflow-prometheus:9090
+    isDefault: true
+    access: proxy