feat(problem): 新增标签管理接口

列表带题目数，重命名撞名时自动合并并返回受影响题数，删除后清标签缓存。

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

problem/urls/admin.py

@@ -11,6 +11,7 @@ from ..views.admin import (
     SQLTestCasePreviewAPI,
     SQLTestCaseScriptsAPI,
     StuckProblemsAPI,
+    TagAdminAPI,
     TestCaseAPI,
     TopACTrendAPI,
 )
@@ -25,6 +26,7 @@ urlpatterns = [
     path("problem/stuck", StuckProblemsAPI.as_view()),
     path("problem/top_ac_trend", TopACTrendAPI.as_view()),
     path("problem/flowchart", ProblemFlowchartAIGen.as_view()),
+    path("problem/tag", TagAdminAPI.as_view()),
     path("contest/problem", ContestProblemAPI.as_view()),
     path("contest_problem/make_public", MakeContestProblemPublicAPIView.as_view()),
     path("contest/add_problem_from_public", AddContestProblemAPI.as_view()),