feat: Add user registration endpoint with JWT token generation and update API documentation

backend/internal/api/routes/router.go

@@ -18,6 +18,7 @@ func SetupRouter(r *gin.Engine) {
 
 	// Public routes
 	r.POST("/auth/login", userHandler.Login)
+	r.POST("/auth/register", userHandler.Register)
 
 	// API routes (protected)
 	api := r.Group("/api")