feat(admin): Phase 1 注册邀请码逻辑 — 码必填 + kind 分流 + validate-invite + 注册页码优先重做

后端:Invitation.kind(join_team/create_team)+ team 可空 + migration;RegisterSerializer 邀请码必填,
create_team 开新团队当 owner+发试用额度、join_team 幂等入伙不重复发额度,删「空码开团队」;
validate-invite 免登录验码端点;团管发码恒 join_team;InvitationSerializer 增 kind/team_name。
前端:注册页码优先(验过才展开;create_team 填团队名 / join_team 只读团队名,不暴露团队列表)、
邀请链接 ?invite= 自动验码、api.validateInvite、验码行/状态提示样式(仅 token)。
测试:accounts 22/22 单测过(RegisterInviteCodeTests + ValidateInviteTests);
无头 e2e _admin-p1.mjs 全断言过 + 0 console error;tsc+build 绿。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

core/backend/apps/accounts/migrations/0005_invitation_kind_alter_invitation_team.py

@@ -0,0 +1,24 @@
+# Generated by Django 5.1.15 on 2026-06-19 12:54
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('accounts', '0004_user_is_platform_admin_adminauditlog'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='invitation',
+            name='kind',
+            field=models.CharField(choices=[('join_team', 'Join Team'), ('create_team', 'Create Team')], default='join_team', max_length=16),
+        ),
+        migrations.AlterField(
+            model_name='invitation',
+            name='team',
+            field=models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.CASCADE, related_name='invitations', to='accounts.team'),
+        ),
+    ]