Fix elevation overlap, add device images, rack merge, and size override

- Fix elevation rendering bug: device blocks were drawn MARGIN px
  further right than the rack frame/unit slots, causing them to
  overlap and extend past the right border. All three now share one
  RACK_X origin.
- Show the device type's front/rear image inside each elevation block
  (clipped to the block, translucent label bar on top for legibility)
  when one is set on the device type; falls back to a plain colour
  block with text otherwise.
- Add "Merge" action to combine two concepts into one taller one (e.g.
  two half-height racks stacked into a full-height rack): the top
  concept's devices are shifted up by the bottom concept's height,
  neither source is modified. Available from the plugin menu, from a
  concept's detail page, and via POST /concepts/merge/.
- Copying a real rack into a concept can now target a different width,
  height and/or starting unit than the source rack; devices that no
  longer fit at the chosen size are skipped and reported as warnings
  instead of aborting the copy.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

netbox_rack_concept/api/serializers.py

@@ -4,6 +4,7 @@ from dcim.api.serializers import (
     DeviceRoleSerializer, DeviceTypeSerializer, LocationSerializer, PlatformSerializer, RackRoleSerializer,
     RackSerializer, RackTypeSerializer, SiteSerializer,
 )
+from dcim.choices import RackWidthChoices
 from dcim.models import Location, Rack, Site
 from netbox.api.serializers import NetBoxModelSerializer
 from tenancy.api.serializers import TenantGroupSerializer, TenantSerializer
@@ -93,6 +94,9 @@ class RackCopySerializer(serializers.Serializer):
     description = serializers.CharField(max_length=200, required=False, allow_blank=True, default='')
     include_devices = serializers.BooleanField(default=True)
     include_device_names = serializers.BooleanField(default=True)
+    override_width = serializers.ChoiceField(choices=RackWidthChoices, required=False, allow_null=True, default=None)
+    override_u_height = serializers.IntegerField(min_value=1, max_value=100, required=False, allow_null=True, default=None)
+    override_starting_unit = serializers.IntegerField(min_value=1, required=False, allow_null=True, default=None)
 
 
 class RackConceptCloneSerializer(serializers.Serializer):
@@ -103,6 +107,24 @@ class RackConceptCloneSerializer(serializers.Serializer):
     include_devices = serializers.BooleanField(default=True)
 
 
+class MergeConceptsSerializer(serializers.Serializer):
+    """POST body for stacking two concepts into one taller one."""
+    concept_bottom = serializers.PrimaryKeyRelatedField(queryset=RackConcept.objects.all())
+    concept_top = serializers.PrimaryKeyRelatedField(queryset=RackConcept.objects.all())
+    name = serializers.CharField(max_length=100)
+    status = serializers.ChoiceField(choices=ConceptStatusChoices, default=ConceptStatusChoices.STATUS_DRAFT)
+    tenant = serializers.PrimaryKeyRelatedField(queryset=Tenant.objects.all(), required=False, allow_null=True)
+    width = serializers.ChoiceField(choices=RackWidthChoices, required=False, allow_null=True, default=None)
+    u_height = serializers.IntegerField(min_value=1, max_value=100, required=False, allow_null=True, default=None)
+    starting_unit = serializers.IntegerField(min_value=1, default=1)
+    description = serializers.CharField(max_length=200, required=False, allow_blank=True, default='')
+
+    def validate(self, attrs):
+        if attrs['concept_bottom'].pk == attrs['concept_top'].pk:
+            raise serializers.ValidationError('concept_bottom and concept_top must be different concepts.')
+        return attrs
+
+
 class ConceptDeploySerializer(serializers.Serializer):
     """POST body for materialising a concept."""
     name_pattern = serializers.CharField(max_length=100)