refactor: [shitty claude AI first try] restructure server and user services, add new test cases, and improve error handling

internal/server/fx.go

@@ -0,0 +1,12 @@
+package server
+
+import "go.uber.org/fx"
+
+// Module provides the server dependencies and lifecycle management
+var Module = fx.Options(
+	fx.Provide(
+		NewFiberApp,
+		NewAPIRouter,
+	),
+	fx.Invoke(StartServer),
+)