feat: Add API key middleware and update configuration to support API key validation

backend/internal/api/middleware/api_key_middleware.go

@@ -0,0 +1,35 @@
+package middleware
+
+import (
+	"github.com/gin-gonic/gin"
+	"github.com/timetracker/backend/internal/api/utils"
+	"github.com/timetracker/backend/internal/config"
+)
+
+// APIKeyMiddleware checks for a valid API key if configured
+func APIKeyMiddleware(cfg *config.Config) gin.HandlerFunc {
+	return func(c *gin.Context) {
+		// Skip if no API key is configured
+		if cfg.APIKey == "" {
+			c.Next()
+			return
+		}
+
+		// Get API key from header
+		apiKey := c.GetHeader("X-API-Key")
+		if apiKey == "" {
+			utils.UnauthorizedResponse(c, "API key is required")
+			c.Abort()
+			return
+		}
+
+		// Validate API key
+		if apiKey != cfg.APIKey {
+			utils.UnauthorizedResponse(c, "Invalid API key")
+			c.Abort()
+			return
+		}
+
+		c.Next()
+	}
+}