feat(maintenance): show maintenance page when the API is in maintenance

The backend now answers 503 with code MAINTENANCE_MODE while maintenance is
on. Without this change a visitor got a red error toast over a broken page
client-side, and a silently empty page server-side, because fetchReq discards
the status and returns null on any failure.

- lib/maintenance.js detects the state by BOTH status 503 and the error code;
  a bare 503 can come from a reverse proxy and is not maintenance
- The axios interceptor checks it before the 401 branch, so a maintenance
  response never triggers the refresh-token path or logs the user out
- fetchReq redirects to /maintenance, with a silentMaintenance opt-out used by
  getStateInfo: that one runs inside generateMetadata and while rendering the
  maintenance page itself, where a redirect is either ineffective or loops
- redirect() works by throwing, so the try/catch blocks in the doctors,
  clinics and specialties pages now rethrow NEXT_REDIRECT instead of
  swallowing it
- clinicApi.js handles 503 too; it previously rendered maintenance as a clinic
  with zero doctors
- The page reuses the existing 404 design and is marked noindex

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/getStateInfo.js

@@ -24,8 +24,12 @@ async function fetchSiteContext(host) {
 
   let value = null;
   try {
+    // این تابع از generateMetadata و از خودِ صفحه‌ی تعمیرات هم صدا زده می‌شود؛
+    // ریدایرکت اینجا یا بی‌اثر است یا حلقه می‌سازد.
     const json = await fetchReq(
-      `${API_URL}/api/v1/site-context?domain=${encodeURIComponent(host)}`
+      `${API_URL}/api/v1/site-context?domain=${encodeURIComponent(host)}`,
+      undefined,
+      { silentMaintenance: true }
     );
     if (json?.data?.type === "representation") {
       value = json.data.representation; // { uuid, full_name, is_global }