feat: add Teacher Admin role to four-tier permission system

Introduces a four-tier role system: Regular User → Student Admin →
Teacher Admin → Super Admin. Teacher Admin can manage own contests,
problemsets, and view classroom data. Student Admin (renamed from Admin)
retains problem management only.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

contest/views/oj.py

@@ -124,7 +124,7 @@ class ContestRankAPI(APIView):
         return (
             ACMContestRank.objects.filter(
                 contest=self.contest,
-                user__admin_type=AdminType.REGULAR_USER,
+                user__admin_type__in=[AdminType.REGULAR_USER, AdminType.STUDENT_ADMIN],
                 user__is_disabled=False,
             )
             .select_related("user")