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>

app/specialties/[slug]/page.js

@@ -9,6 +9,7 @@ import { resolveCityDisplayName } from "@/lib/domainHelpers";
 import { buildSpecialtyFaq, buildSpecialtyIntro } from "@/lib/specialtyContent";
 import { safeJsonLd } from "@/lib/sanitize";
 import { fetchReq } from "@/lib/req";
+import { isNextRedirectError } from "@/lib/maintenance";
 
 const API_URL = process.env.NEXT_PUBLIC_API_URL;
 const DOCTOR_LIMIT = 12;
@@ -32,7 +33,9 @@ const getDoctors = cache(async (specialtyId, cityId) => {
       items: res?.data?.data ?? res?.data ?? [],
       total: Number(res?.data?.meta?.totalRecords ?? res?.meta?.totalRecords ?? 0),
     };
-  } catch {
+  } catch (error) {
+    // ریدایرکت نکست با throw کار می‌کند؛ بدون این rethrow، ریدایرکت حالت تعمیرات بلعیده می‌شود.
+    if (isNextRedirectError(error)) throw error;
     return { items: [], total: 0 };
   }
 });