feat: RRBEC Local Server - Go backend with Django sync

- Implement local-first architecture with SQLite
- Add bidirectional sync with Django via ChangeLog
- JWT authentication with auto-refresh token
- REST API for products, orders, commands, payments
- Stock management with automatic deduction

rrbec_server/internal/api/middleware.go

@@ -0,0 +1,24 @@
+package api
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+)
+
+// SimpleAuthMiddleware checks for a "User-ID" header for simplicity as requested.
+// This can be expanded to use JWT or Cookies if needed.
+func SimpleAuthMiddleware() gin.HandlerFunc {
+	return func(c *gin.Context) {
+		userID := c.GetHeader("X-User-ID")
+		if userID == "" {
+			c.JSON(http.StatusUnauthorized, gin.H{"error": "Authentication required"})
+			c.Abort()
+			return
+		}
+		
+		// Map the user ID to the context for later use
+		c.Set("userID", userID)
+		c.Next()
+	}
+}