feat: implement JWT authentication middleware and handlers

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/middleware/auth.go

@@ -0,0 +1,71 @@
+package middleware
+
+import (
+	"net/http"
+	"strings"
+	"time"
+
+	"github.com/gin-gonic/gin"
+	"github.com/golang-jwt/jwt/v5"
+)
+
+type Claims struct {
+	UserID string `json:"user_id"`
+	jwt.RegisteredClaims
+}
+
+var jwtSecret = []byte("gitm-default-secret")
+
+func SetJWTSecret(secret string) {
+	jwtSecret = []byte(secret)
+}
+
+func GenerateToken(userID string) (string, error) {
+	claims := Claims{
+		UserID: userID,
+		RegisteredClaims: jwt.RegisteredClaims{
+			ExpiresAt: jwt.NewNumericDate(time.Now().Add(24 * time.Hour)),
+			IssuedAt:  jwt.NewNumericDate(time.Now()),
+		},
+	}
+	token := jwt.NewWithClaims(jwt.SigningMethodHS256, claims)
+	return token.SignedString(jwtSecret)
+}
+
+func ValidateToken(tokenString string) (*Claims, error) {
+	token, err := jwt.ParseWithClaims(tokenString, &Claims{}, func(token *jwt.Token) (interface{}, error) {
+		return jwtSecret, nil
+	})
+	if err != nil {
+		return nil, err
+	}
+	if claims, ok := token.Claims.(*Claims); ok && token.Valid {
+		return claims, nil
+	}
+	return nil, jwt.ErrSignatureInvalid
+}
+
+func AuthMiddleware() gin.HandlerFunc {
+	return func(c *gin.Context) {
+		authHeader := c.GetHeader("Authorization")
+		if authHeader == "" {
+			c.JSON(http.StatusUnauthorized, gin.H{"error": "Authorization header required"})
+			c.Abort()
+			return
+		}
+		parts := strings.SplitN(authHeader, " ", 2)
+		if len(parts) != 2 || parts[0] != "Bearer" {
+			c.JSON(http.StatusUnauthorized, gin.H{"error": "Invalid authorization format"})
+			c.Abort()
+			return
+		}
+		claims, err := ValidateToken(parts[1])
+		if err != nil {
+			c.JSON(http.StatusUnauthorized, gin.H{"error": "Invalid token"})
+			c.Abort()
+			return
+		}
+		c.Set("user_id", claims.UserID)
+		c.Next()
+	}
+}