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/pkg/resp/resp.go

@@ -0,0 +1,47 @@
+package resp
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+)
+
+// Error 按 OpenAI 格式返回错误
+func Error(c *gin.Context, status int, message string) {
+	c.AbortWithStatusJSON(status, gin.H{
+		"error": gin.H{
+			"message": message,
+			"type":    "api_error",
+			"param":   nil,
+			"code":    nil,
+		},
+	})
+}
+
+// ErrorWithType 按 OpenAI 格式返回带类型的错误
+func ErrorWithType(c *gin.Context, status int, errType, code, message string) {
+	c.AbortWithStatusJSON(status, gin.H{
+		"error": gin.H{
+			"message": message,
+			"type":    errType,
+			"param":   nil,
+			"code":    code,
+		},
+	})
+}
+
+// ErrorAsAnthropic 按 Anthropic 格式返回错误
+func ErrorAsAnthropic(c *gin.Context, status int, errType, message string) {
+	c.AbortWithStatusJSON(status, gin.H{
+		"type": "error",
+		"error": gin.H{
+			"type":    errType,
+			"message": message,
+		},
+	})
+}
+
+// OK 返回成功 JSON
+func OK(c *gin.Context, data interface{}) {
+	c.JSON(http.StatusOK, data)
+}