fix: configure admin client with service role options and better error logging

- Add auth options (autoRefreshToken: false, persistSession: false) to
  createAdminClient so service role key works correctly with supabase-js
- Return actual Supabase error message in candidature API for debugging

https://claude.ai/code/session_01H2aRGDaKgarPvhay2HxN6Y

lib/supabase/server.ts

@@ -70,6 +70,12 @@ export const createClient = async () => {
 export const createAdminClient = () => {
   return createSupabaseClient<Database>(
     process.env.NEXT_PUBLIC_SUPABASE_URL!,
-    process.env.SUPABASE_SERVICE_ROLE_KEY!
+    process.env.SUPABASE_SERVICE_ROLE_KEY!,
+    {
+      auth: {
+        autoRefreshToken: false,
+        persistSession: false,
+      },
+    }
   );
 };