Add netbox_rack_concept plugin: rack blueprint planning

Introduces a NetBox 4.6 plugin for planning rack layouts (RackConcept,
ConceptDevice) without creating real dcim.Rack/dcim.Device objects, so
blueprints never appear in rack/device lists, utilization reports, or
IPAM.

Features:
- Copy paths: real rack -> concept, concept -> concept (clone), and
  concept -> one or more real racks (deploy, with name patterning and
  dry-run support)
- Manual concept creation via a dedicated "Rack Concepts" nav menu
- Tenant and tenant group assignment on concepts and planned devices
- Partial-width device placement (1/1-1/4) with slot-based collision
  detection, matching netbox_utilities' partial-width feature
- Custom drag-and-drop elevation (netbox-reorder-rack operates on real
  Device querysets, which concepts don't have) with server-side
  validation of every move
- Full REST API, filtering, CSV import, bulk edit, search integration
- Optional, config-driven interop with netbox_utilities (partial-width
  field mapping) and netbox-reorder-rack, degrading gracefully when
  either plugin is absent

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

netbox_rack_concept/template_content.py

@@ -0,0 +1,37 @@
+from django.conf import settings
+
+from netbox.plugins import PluginTemplateExtension
+
+from .models import RackConcept
+
+
+def _enabled():
+    return settings.PLUGINS_CONFIG.get('netbox_rack_concept', {}).get('enable_rack_tab', True)
+
+
+class RackConceptButtons(PluginTemplateExtension):
+    """Adds "Copy to concept" to the button row of a rack, and lists derived concepts."""
+    models = ['dcim.rack']
+
+    def buttons(self):
+        if not _enabled():
+            return ''
+        if not self.context['request'].user.has_perm('netbox_rack_concept.add_rackconcept'):
+            return ''
+        return self.render('netbox_rack_concept/inc/rack_copy_button.html', extra_context={
+            'rack': self.context['object'],
+        })
+
+    def right_page(self):
+        if not _enabled():
+            return ''
+        rack = self.context['object']
+        concepts = RackConcept.objects.restrict(self.context['request'].user, 'view').filter(source_rack=rack)
+        if not concepts:
+            return ''
+        return self.render('netbox_rack_concept/inc/rack_concepts_panel.html', extra_context={
+            'concepts': concepts,
+        })
+
+
+template_extensions = [RackConceptButtons]