Log account delete and subscription changes in UserAuthEvent

Add account_deleted, subscription_started, and subscription_updated
event types. Soft-delete, Checkout/Backer assign, and Stripe portal
lifecycle webhooks write audit rows visible on the user admin.

README.md

@@ -190,7 +190,7 @@ Push/merge to `master` auto-deploys **beta** only. Prod requires the Gitea
 | Auth | JWT (authenticated user only; always deletes `request.user`) |
 | Optional body | `{ "refresh_token": "<current refresh>" }` |
 | Success | `200` `{ "detail": "Account deleted.", "deleted": true }` |
-| Effects | Sets `deleted=True`, `is_active=False`; soft-deletes conversations; blacklists outstanding refresh tokens |
+| Effects | Sets `deleted=True`, `is_active=False`; soft-deletes conversations; blacklists outstanding refresh tokens; logs `UserAuthEvent` `account_deleted` |
 | Staff | Staff/superuser self-delete rejected (`400`, `code=staff_forbidden`) |
 | Privacy v1 | Soft-delete only (no anonymization / hard purge) |
 
@@ -205,6 +205,10 @@ Plan change and cancel stay on Stripe Customer Portal
 `GET /api/finance/subscription/` includes `cancel_at_period_end` and
 `current_period_end` for Account UI messaging.
 
+Subscription audit (`UserAuthEvent` on the user admin):
+- `subscription_started` — first active plan (Checkout, Backer redeem, admin assign)
+- `subscription_updated` — plan/status/cancel-at-period-end changes (portal + webhooks)
+
 ## Security note
 
 Secrets previously hardcoded in `settings.py` (email password, captcha, Django