🎭 飞行侠实现：多身份登录系统

核心功能:
- 用户模型扩展：linked_agents 字段存储绑定龙虾
- 登录 API 支持 3 种模式：human_only / agent_only / both
- 龙虾管理 API：绑定/解绑/列表
- 扫描本机龙虾 API：从注册实例获取

API 端点:
- POST /api/v1/auth/login/ - 支持 login_mode 和 selected_agent_id
- GET  /api/v1/user/linked-agents/ - 获取绑定龙虾
- POST /api/v1/user/linked-agents/ - 添加绑定龙虾
- DELETE /api/v1/user/linked-agents/{id}/ - 移除龙虾
- GET  /api/v1/user/scan-local-agents/ - 扫描本机龙虾

登录模式:
1. human_only - 纯人类身份（1 个座位）
2. agent_only - 纯龙虾身份（1 个座位）
3. both - 双重身份（2 个座位）

测试:
- test_multi_identity.py: 完整测试通过

使用场景:
- 普通用户参会 → human_only
- 龙虾独立参会 → agent_only
- 用户带龙虾助理 → both

backend/users/migrations/0001_initial.py

@@ -0,0 +1,134 @@
+# Generated by Django 6.0.3 on 2026-04-04 04:52
+
+import django.contrib.auth.models
+import django.contrib.auth.validators
+import django.utils.timezone
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    initial = True
+
+    dependencies = [
+        ("auth", "0012_alter_user_first_name_max_length"),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name="User",
+            fields=[
+                (
+                    "id",
+                    models.BigAutoField(
+                        auto_created=True,
+                        primary_key=True,
+                        serialize=False,
+                        verbose_name="ID",
+                    ),
+                ),
+                ("password", models.CharField(max_length=128, verbose_name="password")),
+                (
+                    "last_login",
+                    models.DateTimeField(
+                        blank=True, null=True, verbose_name="last login"
+                    ),
+                ),
+                (
+                    "is_superuser",
+                    models.BooleanField(
+                        default=False,
+                        help_text="Designates that this user has all permissions without explicitly assigning them.",
+                        verbose_name="superuser status",
+                    ),
+                ),
+                (
+                    "username",
+                    models.CharField(
+                        error_messages={
+                            "unique": "A user with that username already exists."
+                        },
+                        help_text="Required. 150 characters or fewer. Letters, digits and @/./+/-/_ only.",
+                        max_length=150,
+                        unique=True,
+                        validators=[
+                            django.contrib.auth.validators.UnicodeUsernameValidator()
+                        ],
+                        verbose_name="username",
+                    ),
+                ),
+                (
+                    "first_name",
+                    models.CharField(
+                        blank=True, max_length=150, verbose_name="first name"
+                    ),
+                ),
+                (
+                    "last_name",
+                    models.CharField(
+                        blank=True, max_length=150, verbose_name="last name"
+                    ),
+                ),
+                (
+                    "is_staff",
+                    models.BooleanField(
+                        default=False,
+                        help_text="Designates whether the user can log into this admin site.",
+                        verbose_name="staff status",
+                    ),
+                ),
+                (
+                    "is_active",
+                    models.BooleanField(
+                        default=True,
+                        help_text="Designates whether this user should be treated as active. Unselect this instead of deleting accounts.",
+                        verbose_name="active",
+                    ),
+                ),
+                (
+                    "date_joined",
+                    models.DateTimeField(
+                        default=django.utils.timezone.now, verbose_name="date joined"
+                    ),
+                ),
+                ("email", models.EmailField(max_length=254, unique=True)),
+                ("created_at", models.DateTimeField(auto_now_add=True)),
+                (
+                    "linked_agents",
+                    models.JSONField(
+                        blank=True, default=list, verbose_name="绑定的龙虾"
+                    ),
+                ),
+                (
+                    "groups",
+                    models.ManyToManyField(
+                        blank=True,
+                        help_text="The groups this user belongs to. A user will get all permissions granted to each of their groups.",
+                        related_name="user_set",
+                        related_query_name="user",
+                        to="auth.group",
+                        verbose_name="groups",
+                    ),
+                ),
+                (
+                    "user_permissions",
+                    models.ManyToManyField(
+                        blank=True,
+                        help_text="Specific permissions for this user.",
+                        related_name="user_set",
+                        related_query_name="user",
+                        to="auth.permission",
+                        verbose_name="user permissions",
+                    ),
+                ),
+            ],
+            options={
+                "verbose_name": "用户",
+                "verbose_name_plural": "用户",
+                "db_table": "users",
+            },
+            managers=[
+                ("objects", django.contrib.auth.models.UserManager()),
+            ],
+        ),
+    ]