refactor: move app entrypoint to ./cmd/server

cmd/server/main.go

@@ -0,0 +1,15 @@
+package main
+
+import (
+	"log"
+	"time"
+)
+
+func main() {
+	log.Println("Starting backoffice-api server...")
+	log.Println("Simulating a running server...")
+	// do nothing, simulate a running server
+	for {
+		time.Sleep(1 * time.Second)
+	}
+}