feat(site_domain): restore access_key enforcement for FQDN lookups

- api_crud_v3: strip falsy access_key values; restrict keyless queries
  to public domains (both site_access_key and site_domain_access_key
  must be NULL/empty); 75-line recursive block replaced with ~16 lines
- lib_sql_search: expand virtual 'access_key' field into priority SQL —
  site_access_key first, site_domain_access_key as fallback
- cms.py: add site_domain_access_key to site_domain searchable_fields
- docs: update frontend guide with access key behavior and examples
- e2e test: expand to cover all valid/invalid access key scenarios (15/15)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/object_definitions/cms.py

@@ -124,7 +124,7 @@ cms_obj_li = {
         'searchable_fields': [
             'id', 'account_id', 'site_id',
             'id_random', 'account_id_random', 'site_id_random',
-            'fqdn', 'access_key', 'site_access_key',
+            'fqdn', 'access_key', 'site_access_key', 'site_domain_access_key',
             'enable', 'created_on', 'updated_on'
         ],
     },