feat(core): Wave 3.5 邀请制后端 — Invitation 模型 + 端点 + register 改造(只生成迁移不apply)

- Invitation 模型:team/code(TEAM-XXXX-XXXX唯一)/role/email/月额度/created_by/expires_at(7天)/
  used_by/used_at/status(pending/used/revoked/expired);迁移 accounts/0003(未 apply 远程库,rule 3)
- RegisterSerializer 加 invite_code:有效码→幂等 get_or_create 成员+标used+跳过额度发放;
  无码→开新团队当 owner+试用额度(原逻辑);邀请 role 不许 OWNER 降级 ADMIN;不存在/已用/过期→400
- 端点 /api/auth/team/invitations/(GET列/POST生成)+ <id>/revoke/;权限复用 can_manage_team(超管/团管)

验证: django check 0;apps.accounts 测试 5/5 OK —— 含「凭码加入同团队/角色额度对/不重复发额度/
  未知码400/普通成员生成403」。认证定调:username 为主标识、不用邮箱注册。

⚠️ Invitation 迁移需用户低峰 apply 到远程 MySQL,前端生成码 e2e 才能跑(逻辑已被测试证)。

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

core/backend/apps/accounts/migrations/0003_invitation.py

@@ -0,0 +1,38 @@
+# Generated by Django 5.1.15 on 2026-06-19 04:29
+
+import apps.accounts.models
+import django.db.models.deletion
+import uuid
+from django.conf import settings
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('accounts', '0002_loginsession_userpreference'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='Invitation',
+            fields=[
+                ('id', models.UUIDField(default=uuid.uuid4, editable=False, primary_key=True, serialize=False)),
+                ('created_at', models.DateTimeField(auto_now_add=True)),
+                ('updated_at', models.DateTimeField(auto_now=True)),
+                ('code', models.CharField(default=apps.accounts.models._gen_invite_code, max_length=32, unique=True)),
+                ('role', models.CharField(choices=[('owner', 'Owner'), ('admin', 'Admin'), ('member', 'Member'), ('viewer', 'Viewer')], default='member', max_length=24)),
+                ('email', models.EmailField(blank=True, max_length=254)),
+                ('monthly_credit_limit', models.DecimalField(decimal_places=2, default=0, max_digits=12)),
+                ('expires_at', models.DateTimeField(default=apps.accounts.models._default_invite_expiry)),
+                ('used_at', models.DateTimeField(blank=True, null=True)),
+                ('status', models.CharField(choices=[('pending', 'Pending'), ('used', 'Used'), ('revoked', 'Revoked'), ('expired', 'Expired')], default='pending', max_length=16)),
+                ('created_by', models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='created_invitations', to=settings.AUTH_USER_MODEL)),
+                ('team', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='invitations', to='accounts.team')),
+                ('used_by', models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='used_invitations', to=settings.AUTH_USER_MODEL)),
+            ],
+            options={
+                'indexes': [models.Index(fields=['team', 'status'], name='accounts_in_team_id_b48ad1_idx')],
+            },
+        ),
+    ]