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/choices.py

@@ -0,0 +1,61 @@
+from utilities.choices import ChoiceSet
+
+
+class ConceptStatusChoices(ChoiceSet):
+    """Lifecycle of a rack blueprint."""
+    key = 'RackConcept.status'
+
+    STATUS_DRAFT = 'draft'
+    STATUS_REVIEW = 'review'
+    STATUS_APPROVED = 'approved'
+    STATUS_ARCHIVED = 'archived'
+
+    CHOICES = [
+        (STATUS_DRAFT, 'Draft', 'gray'),
+        (STATUS_REVIEW, 'In review', 'orange'),
+        (STATUS_APPROVED, 'Approved', 'green'),
+        (STATUS_ARCHIVED, 'Archived', 'red'),
+    ]
+
+
+class ConceptDeviceStatusChoices(ChoiceSet):
+    """
+    What a planned device means. This is deliberately *not* dcim's device status:
+    a blueprint describes intent, not operational state.
+    """
+    key = 'ConceptDevice.status'
+
+    STATUS_PLANNED = 'planned'
+    STATUS_EXISTING = 'existing'
+    STATUS_OPTIONAL = 'optional'
+    STATUS_RESERVED = 'reserved'
+
+    CHOICES = [
+        (STATUS_PLANNED, 'Planned', 'blue'),
+        (STATUS_EXISTING, 'Existing', 'green'),
+        (STATUS_OPTIONAL, 'Optional', 'cyan'),
+        (STATUS_RESERVED, 'Reserved space', 'gray'),
+    ]
+
+
+class WidthFractionChoices(ChoiceSet):
+    """
+    Horizontal subdivision of a rack unit, mirroring the partial-width feature of
+    netbox_utilities. The value is the denominator: 1 = full width, 2 = half, ...
+    """
+    FRACTION_FULL = 1
+    FRACTION_HALF = 2
+    FRACTION_THIRD = 3
+    FRACTION_QUARTER = 4
+
+    CHOICES = (
+        (FRACTION_FULL, 'Full width'),
+        (FRACTION_HALF, '1/2'),
+        (FRACTION_THIRD, '1/3'),
+        (FRACTION_QUARTER, '1/4'),
+    )
+
+
+# Least common multiple of all supported denominators. The elevation is laid out
+# on this many lanes so that every fraction lands on an integer lane boundary.
+LANE_COUNT = 12