fix: read backbone_code from multipart form in tower controller

CreateTower and UpdateTower multipart handlers were building the DTO
without reading backbone_code from PostForm, so the field was always nil
even when the frontend sent it. Added c.PostForm("backbone_code") parsing
in both handlers.

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

delivery/controller/tower_controller.go

@@ -185,6 +185,11 @@ func (tc *TowerController) CreateTower() gin.HandlerFunc {
             externalTower = &parsedExternalTower
         }
 
+        var backboneCode *string
+        if bc := c.PostForm("backbone_code"); bc != "" {
+            backboneCode = &bc
+        }
+
         // Create DTO
         towerDTO := req.TowerDTO{
             DeviceID:      deviceID,
@@ -192,6 +197,7 @@ func (tc *TowerController) CreateTower() gin.HandlerFunc {
             Longitude:     longitude,
             Latitude:      latitude,
             ExternalTower: externalTower,
+            BackboneCode:  backboneCode,
         }
 
         // Get multiple image files
@@ -322,6 +328,10 @@ func (tc *TowerController) UpdateTower() gin.HandlerFunc {
             towerUpdateDTO.ImageURL = &imageURL
         }
 
+        if backboneCode := c.PostForm("backbone_code"); backboneCode != "" {
+            towerUpdateDTO.BackboneCode = &backboneCode
+        }
+
         // Get multiple image files
         form := c.Request.MultipartForm
         imageFiles := form.File["images"] // Support multiple images