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>

documentation/GUIDE__AE_API_V3_for_Frontend.md

@@ -44,6 +44,37 @@ When the frontend first loads and doesn't know the `account_id`, it performs a "
 *   Returns 200 + a list containing the `account_id` (random string ID) and `site_id` (random string ID).
 *   ** デザイン Choice:** If the domain is not found, it returns **200 OK with an empty list `[]`**. It is NOT a 404.
 
+> **Access Key Support**
+>
+> Some client deployments restrict their domain via an access key passed in the browser URL (e.g. `?key=abc123`). The frontend reads this param and forwards it as `access_key` in the POST body.
+>
+> **How to pass the key:**
+> ```json
+> {
+>   "and": [
+>     { "field": "fqdn", "op": "eq", "value": "client.example.com" },
+>     { "field": "access_key", "op": "eq", "value": "abc123" }
+>   ]
+> }
+> ```
+> If `key` is absent, empty, or falsy — **omit `access_key` from the payload entirely**. Do not send `"access_key": ""`.
+>
+> **Server behavior:**
+> - `site_access_key` (site-level key) takes priority. If set, all domains under that site require it.
+> - `site_domain_access_key` (domain-level key) is used as fallback when `site_access_key` is not set.
+> - A domain is **public** only when **both** key columns are NULL/empty.
+> - Falsy `access_key` values are ignored server-side as a safety net.
+> - Match → `200` with the record. No match → `200` with empty list `[]`.
+> - Do **not** use `access_code_kv_json` for this — that field is for UI features only.
+>
+> | Browser URL | `access_key` in payload | Result |
+> |---|---|---|
+> | `https://dev-demo.oneskyit.com` | *(omit)* | ✅ Returns record (public) |
+> | `https://client.example.com/?key=correct` | `"correct"` | ✅ Returns record |
+> | `https://client.example.com/` | *(omit)* | ❌ Empty (key required) |
+> | `https://client.example.com/?key=wrong` | `"wrong"` | ❌ Empty (wrong key) |
+> | `https://client.example.com/?key=` | *(omit — strip empty)* | ❌ Empty (key required) |
+>
 ---
 
 ## 3. Standard CRUD Patterns