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/urls.py

@@ -8,8 +8,10 @@ from .views import (
     me,
     preferences,
     register,
+    revoke_invitation,
     revoke_login_session,
     revoke_other_sessions,
+    team_invitations,
     team_member_detail,
     team_member_password,
     team_members,
@@ -31,4 +33,6 @@ urlpatterns = [
     path("team/members/", team_members, name="team-members"),
     path("team/members/<uuid:member_id>/", team_member_detail, name="team-member-detail"),
     path("team/members/<uuid:member_id>/password/", team_member_password, name="team-member-password"),
+    path("team/invitations/", team_invitations, name="team-invitations"),
+    path("team/invitations/<uuid:invite_id>/revoke/", revoke_invitation, name="team-invitation-revoke"),
 ]