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/doctors/page.js

@@ -3,6 +3,7 @@ import Layout from "@/components/layout/StLayout";
 import { buildDoctorParams } from "@/helper";
 import { getStateInfo } from "@/lib/getStateInfo";
 import { fetchReq } from "@/lib/req";
+import { isNextRedirectError } from "@/lib/maintenance";
 import { listingRobots } from "@/lib/listingRobots";
 
 export async function generateMetadata({ searchParams }) {
@@ -62,6 +63,8 @@ async function Doctors({ searchParams }) {
       params,
     });
   } catch (error) {
+    // ریدایرکت نکست با throw کار می‌کند؛ بدون این rethrow، ریدایرکت حالت تعمیرات بلعیده می‌شود.
+    if (isNextRedirectError(error)) throw error;
     console.error("Error fetching doctors:", error);
   }