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/jwt/jwt.go

@@ -0,0 +1,61 @@
+package jwt
+
+import (
+	"errors"
+	"time"
+
+	gojwt "github.com/golang-jwt/jwt/v5"
+)
+
+type Claims struct {
+	UserID uint64 `json:"user_id"`
+	Name   string `json:"name"`
+	Role   string `json:"role"`
+	gojwt.RegisteredClaims
+}
+
+// GenerateTokenPair 生成 access + refresh token
+func GenerateTokenPair(userID uint64, name, role, secret string, accessExpire, refreshExpire time.Duration) (accessToken, refreshToken string, err error) {
+	accessToken, err = generateToken(userID, name, role, "access", secret, accessExpire)
+	if err != nil {
+		return "", "", err
+	}
+	refreshToken, err = generateToken(userID, name, role, "refresh", secret, refreshExpire)
+	if err != nil {
+		return "", "", err
+	}
+	return
+}
+
+func generateToken(userID uint64, name, role, tokenType, secret string, expire time.Duration) (string, error) {
+	now := time.Now()
+	claims := Claims{
+		UserID: userID,
+		Name:   name,
+		Role:   role,
+		RegisteredClaims: gojwt.RegisteredClaims{
+			ExpiresAt: gojwt.NewNumericDate(now.Add(expire)),
+			IssuedAt:  gojwt.NewNumericDate(now),
+			NotBefore: gojwt.NewNumericDate(now),
+		},
+	}
+	token := gojwt.NewWithClaims(gojwt.SigningMethodHS256, claims)
+	return token.SignedString([]byte(secret))
+}
+
+// ValidateToken 校验 JWT
+func ValidateToken(tokenString, secret string) (*Claims, error) {
+	token, err := gojwt.ParseWithClaims(tokenString, &Claims{}, func(token *gojwt.Token) (interface{}, error) {
+		if _, ok := token.Method.(*gojwt.SigningMethodHMAC); !ok {
+			return nil, errors.New("unexpected signing method")
+		}
+		return []byte(secret), nil
+	})
+	if err != nil {
+		return nil, err
+	}
+	if claims, ok := token.Claims.(*Claims); ok && token.Valid {
+		return claims, nil
+	}
+	return nil, gojwt.ErrInvalidKey
+}