fix(problem): resolve_tags 的 create 包一层 atomic

外层若在事务里，IntegrityError 会中止整个事务，导致回查抛
TransactionManagementError。当前未开 ATOMIC_REQUESTS 所以还没触发，
但仓库多处用了 transaction.atomic()，属于随时会踩的坑。

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

problem/services.py

@@ -1,5 +1,5 @@
 from django.core.cache import cache
-from django.db import IntegrityError
+from django.db import IntegrityError, transaction
 
 from utils.constants import CacheKey
 
@@ -39,6 +39,9 @@ def resolve_tags(names):
         tag = ProblemTag.objects.filter(name__iexact=name).first()
         if tag is None:
             try:
+                # 包一层 atomic：外层若在事务里，IntegrityError 会中止整个事务，
+                # 导致下面的回查抛 TransactionManagementError
+                with transaction.atomic():
                     tag = ProblemTag.objects.create(name=name)
                 created = True
             except IntegrityError: