refactor(problem): 标签创建逻辑收敛到 services.resolve_tags

原本 4 处题目保存逻辑各自复制了一份 get-or-create，且大小写敏感、有竞态。

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

problem/services.py

@@ -0,0 +1,52 @@
+from django.core.cache import cache
+from django.db import IntegrityError
+
+from utils.constants import CacheKey
+
+from .models import ProblemTag
+
+
+def clear_tag_cache():
+    """标签列表接口按 keyword 分键缓存，标签一变就整批清掉"""
+    cache.delete_pattern(f"{CacheKey.problem_tags}:*")
+
+
+def find_tags(names):
+    """按名字大小写不敏感查已有标签，查不到就跳过，不创建"""
+    tags = []
+    seen = set()
+    for raw in names:
+        name = (raw or "").strip()
+        if not name or name.lower() in seen:
+            continue
+        seen.add(name.lower())
+        tag = ProblemTag.objects.filter(name__iexact=name).first()
+        if tag is not None:
+            tags.append(tag)
+    return tags
+
+
+def resolve_tags(names):
+    """把前端传来的标签名解析成 ProblemTag：去空格、大小写不敏感复用已有标签，没有才新建"""
+    tags = []
+    seen = set()
+    created = False
+    for raw in names:
+        name = (raw or "").strip()
+        if not name or name.lower() in seen:
+            continue
+        seen.add(name.lower())
+        tag = ProblemTag.objects.filter(name__iexact=name).first()
+        if tag is None:
+            try:
+                tag = ProblemTag.objects.create(name=name)
+                created = True
+            except IntegrityError:
+                # 并发下另一个请求刚建好同名标签，回查复用
+                tag = ProblemTag.objects.filter(name__iexact=name).first()
+                if tag is None:
+                    continue
+        tags.append(tag)
+    if created:
+        clear_tag_cache()
+    return tags