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/utils/pointer.go

@@ -0,0 +1,11 @@
+package utils
+
+func ToPtr[T any](v T) *T {
+	return &v
+}
+
+func UpdatePtrField[T any](target *T, value *T) {
+	if value != nil {
+		*target = *value
+	}
+}