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.

Makefile

@@ -2,10 +2,11 @@
 
 BINARY_NAME=openteam
 BUILD_DIR=bin
+BACKEND_DIR=backend
 
 # Build
 build:
-	go build -o $(BUILD_DIR)/$(BINARY_NAME) ./cmd/openteam
+	cd $(BACKEND_DIR) && CGO_ENABLED=1 go build -o ../$(BUILD_DIR)/$(BINARY_NAME) ./cmd/openteam
 
 # Run
 run: build
@@ -13,41 +14,41 @@ run: build
 
 # Development run
 dev:
-	go run ./cmd/openteam
+	cd $(BACKEND_DIR) && go run ./cmd/openteam
 
 # Test
 test:
-	go test ./internal/... -v
+	cd $(BACKEND_DIR) && go test ./internal/... -v
 
 # Test with coverage
 test-cover:
-	go test ./internal/... -coverprofile=coverage.out
-	go tool cover -html=coverage.out -o coverage.html
+	cd $(BACKEND_DIR) && go test ./internal/... -coverprofile=coverage.out
+	cd $(BACKEND_DIR) && go tool cover -html=coverage.out -o coverage.html
 
 # Format code
 fmt:
-	go fmt ./...
+	cd $(BACKEND_DIR) && go fmt ./...
 
 # Lint
 lint:
-	golangci-lint run
+	cd $(BACKEND_DIR) && golangci-lint run
 
 # Clean
 clean:
 	rm -rf $(BUILD_DIR)
-	rm -f coverage.out coverage.html
+	cd $(BACKEND_DIR) && rm -f coverage.out coverage.html
 
 # Tidy dependencies
 tidy:
-	go mod tidy
+	cd $(BACKEND_DIR) && go mod tidy
 
 # Build for Linux
 build-linux:
-	CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build -o $(BUILD_DIR)/$(BINARY_NAME)-linux-amd64 ./cmd/openteam
+	cd $(BACKEND_DIR) && CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build -o ../$(BUILD_DIR)/$(BINARY_NAME)-linux-amd64 ./cmd/openteam
 
 # Build for macOS
 build-mac:
-	CGO_ENABLED=0 GOOS=darwin GOARCH=arm64 go build -o $(BUILD_DIR)/$(BINARY_NAME)-darwin-arm64 ./cmd/openteam
+	cd $(BACKEND_DIR) && CGO_ENABLED=0 GOOS=darwin GOARCH=arm64 go build -o ../$(BUILD_DIR)/$(BINARY_NAME)-darwin-arm64 ./cmd/openteam
 
 # Build all platforms
 build-all: build-linux build-mac