Add account self-delete and subscription lifecycle sync (#34) (#39)

## Summary
- Closes [#34](#34)
- Companion for [chat_web_app#75](ai_ml_operations/chat_web_app#75) (portal cancel/change local sync)
- Soft-delete `DELETE /api/user/` for the authenticated user only: `deleted=True`, `is_active=False`, hide conversations, blacklist outstanding refresh tokens; staff self-delete rejected
- Stripe `customer.subscription.updated` / `deleted` webhooks sync plan status, `cancel_at_period_end`, and `current_period_end`; checkout assigns plan from `metadata.plan_slug`
- **UserAuthEvent audit**: `account_deleted`, `subscription_started` (first active plan), `subscription_updated` (plan/status/cancel changes) — visible on user admin
- Document FE contract in README (endpoint, response, post-delete logout)

## Test plan
- [ ] `uv run python manage.py test chat_backend.tests.test_views_users.CustomUserSelfDeleteTestCase finance.tests`
- [ ] Authenticated `DELETE /api/user/` soft-deletes self, hides conversations, blocks re-login, writes `account_deleted` auth event
- [ ] Checkout / Backer assign writes `subscription_started`; portal cancel/change writes `subscription_updated`
- [ ] Anonymous / staff self-delete rejected; body cannot target another user
- [ ] After portal cancel, webhook sets `cancel_at_period_end` / `canceled` on `GET /finance/subscription/`Reviewed-on: #39

llm_be/finance/models.py

@@ -149,6 +149,15 @@ class UserSubscription(TimeInfoBase):
         default="",
         db_index=True,
     )
+    cancel_at_period_end = models.BooleanField(
+        default=False,
+        help_text="Stripe: subscription will cancel at current_period_end.",
+    )
+    current_period_end = models.DateTimeField(
+        null=True,
+        blank=True,
+        help_text="Stripe billing period end (access remains until then when canceling).",
+    )
     monthly_token_quota_override = models.PositiveIntegerField(
         null=True,
         blank=True,