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/auth/auth.go

@@ -0,0 +1,78 @@
+package auth
+
+import (
+	"errors"
+	"os"
+	"opencatd-open/internal/store"
+	"time"
+
+	"github.com/golang-jwt/jwt/v5"
+)
+
+type Claims struct {
+	UserID uint64 `json:"user_id"`
+	Name   string `json:"name"`
+	Type   string `json:"type"`
+	jwt.RegisteredClaims
+}
+
+type TokenPair struct {
+	AccessToken  string `json:"access_token"`
+	RefreshToken string `json:"refresh_token,omitempty"`
+}
+
+func GenerateTokenPair(user *store.User, secret string, accessExpire, refreshExpire time.Duration) (*TokenPair, error) {
+	accessToken, err := generateToken(user, "access", secret, accessExpire)
+	if err != nil {
+		return nil, err
+	}
+	refreshToken, err := generateToken(user, "refresh", secret, refreshExpire)
+	if err != nil {
+		return nil, err
+	}
+	return &TokenPair{
+		AccessToken:  accessToken,
+		RefreshToken: refreshToken,
+	}, nil
+}
+
+func generateToken(user *store.User, tokenType, secret string, expire time.Duration) (string, error) {
+	now := time.Now()
+	claims := Claims{
+		UserID: user.ID,
+		Name:   user.Username,
+		Type:   tokenType,
+		RegisteredClaims: jwt.RegisteredClaims{
+			ExpiresAt: jwt.NewNumericDate(now.Add(expire)),
+			IssuedAt:  jwt.NewNumericDate(now),
+			NotBefore: jwt.NewNumericDate(now),
+		},
+	}
+	token := jwt.NewWithClaims(jwt.SigningMethodHS256, claims)
+	return token.SignedString([]byte(secret))
+}
+
+func ValidateToken(tokenString, secret string) (*Claims, error) {
+	token, err := jwt.ParseWithClaims(tokenString, &Claims{}, func(token *jwt.Token) (interface{}, error) {
+		if _, ok := token.Method.(*jwt.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, jwt.ErrInvalidKey
+}
+
+// GetSecretKey returns the JWT secret key from environment or config
+func GetSecretKey() string {
+	secret := os.Getenv("SECRET_KEY")
+	if secret == "" {
+		secret = "default-secret-key-change-in-production"
+	}
+	return secret
+}