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/chat_backend/serializers.py

@@ -26,6 +26,17 @@ class MyTokenObtainPairSerializer(TokenObtainPairSerializer):
 
         return token
 
+    def validate(self, attrs):
+        data = super().validate(attrs)
+        if getattr(self.user, "deleted", False):
+            from rest_framework_simplejwt.exceptions import AuthenticationFailed
+
+            raise AuthenticationFailed(
+                "No active account found with the given credentials",
+                code="no_active_account",
+            )
+        return data
+
 
 class CompanySerializer(serializers.ModelSerializer):
     class Meta:
@@ -70,12 +81,18 @@ class CustomUserSerializer(serializers.ModelSerializer):
                 "status": UserSubscription.Status.NONE,
                 "source": UserSubscription.Source.NONE,
                 "needs_checkout": True,
+                "cancel_at_period_end": False,
+                "current_period_end": None,
             }
         return {
             "plan": plan_to_dict(sub.plan) if sub.plan_id else None,
             "status": sub.status,
             "source": sub.source,
             "needs_checkout": needs_checkout(obj),
+            "cancel_at_period_end": bool(sub.cancel_at_period_end),
+            "current_period_end": (
+                sub.current_period_end.isoformat() if sub.current_period_end else None
+            ),
         }