feat(5.20): Custom Fields — plugin-defined fields in Contact UI

- Add CustomFieldDefinition to PluginManifest (text/number/date/select/multiselect/boolean)
- Add GET/PATCH /api/v1/contacts/{id}/custom-fields routes
- Merge plugin definitions with stored values in contacts.custom JSONB
- Add CustomFieldRenderer component (read + edit modes)
- Integrate into ContactDetail (read-only) and ContactEditModal (editable)
- Add custom_fields to pluginStore and active manifests API
- Add useCustomFields/useUpdateCustomFields React Query hooks
- Tests: test_custom_fields.py (9 tests) + CustomFieldRenderer.test.tsx (5 tests)
- i18n keys already present (contacts.customFields)

frontend/src/store/pluginStore.ts

@@ -50,6 +50,17 @@ export interface PluginDashboardWidget {
   permission: string;
 }
 
+export interface PluginCustomFieldDefinition {
+  name: string;
+  label: string;
+  label_key: string;
+  field_type: 'text' | 'number' | 'date' | 'select' | 'multiselect' | 'boolean';
+  options: string[];
+  default_value: any;
+  required: boolean;
+  entity: string;
+}
+
 export interface PluginUiManifest {
   name: string;
   display_name: string;
@@ -60,6 +71,7 @@ export interface PluginUiManifest {
   detail_tabs: PluginDetailTab[];
   settings_pages: PluginSettingsPage[];
   dashboard_widgets: PluginDashboardWidget[];
+  custom_fields: PluginCustomFieldDefinition[];
 }
 
 interface PluginState {
@@ -77,6 +89,7 @@ interface PluginState {
   getDetailTabsForEntity: (entityType: string) => PluginDetailTab[];
   getAllSettingsPages: () => PluginSettingsPage[];
   getAllDashboardWidgets: () => PluginDashboardWidget[];
+  getCustomFieldsForEntity: (entityType: string) => PluginCustomFieldDefinition[];
 }
 
 export const usePluginStore = create<PluginState>((set, get) => ({
@@ -125,4 +138,11 @@ export const usePluginStore = create<PluginState>((set, get) => ({
       .flatMap((m) => m.dashboard_widgets)
       .sort((a, b) => a.order - b.order);
   },
+
+  getCustomFieldsForEntity: (entityType: string) => {
+    const { manifests } = get();
+    return manifests
+      .flatMap((m) => m.custom_fields || [])
+      .filter((cf) => cf.entity === entityType);
+  },
 }));