Fixes #7604: Add filter modifier dropdowns for advanced lookup operators

Implements dynamic filter modifier UI that allows users to select lookup operators
(exact, contains, starts with, regex, negation, empty/not empty) directly in filter
forms without manual URL parameter editing.

Supports filters for all scalar types and strings, as well as some
related object filters. Explicitly does not support filters on fields
that use APIWidget. That has been broken out in to follow up work.

**Backend:**
- FilterModifierWidget: Wraps form widgets with lookup modifier dropdown
- FilterModifierMixin: Auto-enhances filterset fields with appropriate lookups
- Extended lookup support: Adds negation (n), regex, iregex, empty_true/false lookups
- Field-type-aware: CharField gets text lookups, IntegerField gets comparison operators, etc.

**Frontend:**
- TypeScript handler syncs modifier dropdown with URL parameters
- Dynamically updates form field names (serial → serial__ic) on modifier change
- Flexible-width modifier dropdowns with semantic CSS classes

netbox/circuits/forms/filtersets.py

@@ -13,8 +13,11 @@ from netbox.forms import NetBoxModelFilterSetForm, OrganizationalModelFilterSetF
 from tenancy.forms import TenancyFilterForm, ContactModelFilterForm
 from utilities.forms import add_blank_choice
 from utilities.forms.fields import ColorField, DynamicModelMultipleChoiceField, TagFilterField
+from utilities.forms.filterset_mappings import FILTERSET_MAPPINGS
+from utilities.forms.mixins import FilterModifierMixin
 from utilities.forms.rendering import FieldSet
 from utilities.forms.widgets import DatePicker, NumberWithOptions
+from circuits.filtersets import CircuitFilterSet
 
 __all__ = (
     'CircuitFilterForm',
@@ -118,7 +121,7 @@ class CircuitTypeFilterForm(OrganizationalModelFilterSetForm):
     )
 
 
-class CircuitFilterForm(TenancyFilterForm, ContactModelFilterForm, PrimaryModelFilterSetForm):
+class CircuitFilterForm(FilterModifierMixin, TenancyFilterForm, ContactModelFilterForm, NetBoxModelFilterSetForm):
     model = Circuit
     fieldsets = (
         FieldSet('q', 'filter_id', 'tag', 'owner_id'),
@@ -397,3 +400,7 @@ class VirtualCircuitTerminationFilterForm(NetBoxModelFilterSetForm):
         label=_('Provider')
     )
     tag = TagFilterField(model)
+
+
+# Register FilterSet mappings for FilterModifierMixin lookup verification
+FILTERSET_MAPPINGS[CircuitFilterForm] = CircuitFilterSet