feat(problem): 标签名去空格并加大小写不敏感唯一约束

迁移时先合并已存在的重复标签，题目关系转移到保留的那个。

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

problem/migrations/0014_problem_tag_ci_unique.py

@@ -0,0 +1,49 @@
+from collections import defaultdict
+
+from django.db import migrations, models
+from django.db.models.functions import Lower
+
+
+def merge_duplicate_tags(apps, schema_editor):
+    """加唯一约束前，把「去空格+转小写」后同名的标签合并成一个。
+
+    每组保留关联题目数最多的（并列时保留 id 最小的）作为主标签，
+    其余标签下的题目关系转移到主标签后删除。空名标签同样按此规则归并成一条，
+    不做删除，留给管理员在标签管理页处理。
+    """
+    ProblemTag = apps.get_model("problem", "ProblemTag")
+    Problem = apps.get_model("problem", "Problem")
+
+    groups = defaultdict(list)
+    for tag in ProblemTag.objects.all():
+        groups[(tag.name or "").strip().lower()].append(tag)
+
+    for tags in groups.values():
+        counts = {tag.id: Problem.objects.filter(tags=tag).count() for tag in tags}
+        tags.sort(key=lambda tag: (-counts[tag.id], tag.id))
+        primary = tags[0]
+
+        stripped = (primary.name or "").strip()
+        if primary.name != stripped:
+            primary.name = stripped
+            primary.save(update_fields=["name"])
+
+        for duplicate in tags[1:]:
+            for problem in Problem.objects.filter(tags=duplicate):
+                problem.tags.add(primary)
+                problem.tags.remove(duplicate)
+            duplicate.delete()
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("problem", "0013_remove_problem_io_mode_remove_problem_rule_type_and_more"),
+    ]
+
+    operations = [
+        migrations.RunPython(merge_duplicate_tags, migrations.RunPython.noop),
+        migrations.AddConstraint(
+            model_name="problemtag",
+            constraint=models.UniqueConstraint(Lower("name"), name="problem_tag_name_ci_unique"),
+        ),
+    ]