refactor: move backend files to backend/ directory

Reorganize project structure:
- backend/cmd/openteam/ — entry point
- backend/internal/ — core packages
- backend/middleware/ — HTTP middleware
- backend/router/ — route setup
- backend/wire/ — dependency injection
- backend/pkg/ — shared utilities
- backend/go.mod, go.sum — Go module files

Updated Makefile to work from backend/ directory.
Removed old lowercase makefile.

backend/internal/dto/user.go

@@ -0,0 +1,16 @@
+package dto
+
+type User struct {
+	Username string `json:"username" binding:"required,min=3,max=32"`
+	Password string `json:"password" binding:"required,min=4"`
+}
+
+type Auth struct {
+	Token     string `json:"token"`
+	ExpiresIn int64  `json:"expires_in"`
+}
+
+type ChangePassword struct {
+	Password    string `json:"password" binding:"required,min=4"`
+	NewPassword string `json:"newpassword" binding:"required,min=4"`
+}