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/batch.go

@@ -0,0 +1,6 @@
+package dto
+
+type BatchIDRequest struct {
+	UserID *int64  `json:"user_id"`
+	IDs    []int64 `json:"ids" binding:"required"`
+}