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)

app/plugins/registry.py

@@ -760,6 +760,7 @@ class PluginRegistry:
                     "detail_tabs": [tab.model_dump() for tab in m.detail_tabs],
                     "settings_pages": [page.model_dump() for page in m.settings_pages],
                     "dashboard_widgets": [widget.model_dump() for widget in m.dashboard_widgets],
+                    "custom_fields": [cf.model_dump() for cf in m.custom_fields],
                 }
             )
         return manifests