feat: 添加所有 Django apps 的 Serializers

- User Serializers（基础、详情、更新、统计）
- Region Serializers（基础、详情、树形结构）
- Moderation Serializers（版主申请、权限、支持、限制）
- Article Serializers（基础、创建、更新、审核、列表）
- FeaturedService Serializers（基础、创建、更新、审核、列表）
- Interaction Serializers（评论、评分、点赞、收藏）

backend/apps/regions/serializers.py

@@ -0,0 +1,53 @@
+from rest_framework import serializers
+from .models import Region
+
+
+class RegionSerializer(serializers.ModelSerializer):
+    """Serializer for Region model."""
+
+    level_display = serializers.CharField(source='get_level_display', read_only=True)
+    status_display = serializers.CharField(source='get_status_display', read_only=True)
+    parent_name = serializers.CharField(source='parent.name', read_only=True, allow_null=True)
+    children_count = serializers.SerializerMethodField()
+
+    class Meta:
+        model = Region
+        fields = ('id', 'name', 'level', 'level_display', 'parent', 'parent_name',
+                  'status', 'status_display', 'children_count', 'created_at', 'updated_at')
+        read_only_fields = ('id', 'created_at', 'updated_at')
+
+    def get_children_count(self, obj):
+        return obj.children.count()
+
+
+class RegionDetailSerializer(serializers.ModelSerializer):
+    """Detailed serializer for Region model."""
+
+    level_display = serializers.CharField(source='get_level_display', read_only=True)
+    status_display = serializers.CharField(source='get_status_display', read_only=True)
+    parent = RegionSerializer(read_only=True)
+    children = RegionSerializer(many=True, read_only=True)
+    full_path = serializers.SerializerMethodField()
+
+    class Meta:
+        model = Region
+        fields = ('id', 'name', 'level', 'level_display', 'parent', 'children',
+                  'status', 'status_display', 'full_path', 'created_at', 'updated_at')
+        read_only_fields = ('id', 'created_at', 'updated_at')
+
+    def get_full_path(self, obj):
+        return obj.get_full_path()
+
+
+class RegionTreeSerializer(serializers.ModelSerializer):
+    """Serializer for Region tree structure."""
+
+    children = serializers.SerializerMethodField()
+
+    class Meta:
+        model = Region
+        fields = ('id', 'name', 'level', 'status', 'children')
+
+    def get_children(self, obj):
+        children = obj.get_children()
+        return RegionTreeSerializer(children, many=True).data