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

Soft-delete DELETE /api/user/ for authenticated users (hide conversations,
blacklist tokens, block staff self-delete). Sync Stripe portal cancel/change
via subscription.updated/deleted webhooks and expose cancel_at_period_end for
Account UI (chat_web_app#75 companion).

llm_be/finance/views.py

@@ -199,6 +199,12 @@ class SubscriptionMeView(APIView):
             "stripe_subscription_id": (
                 sub.stripe_subscription_id if sub else ""
             ),
+            "cancel_at_period_end": bool(sub.cancel_at_period_end) if sub else False,
+            "current_period_end": (
+                sub.current_period_end.isoformat()
+                if sub and sub.current_period_end
+                else None
+            ),
             "usage": usage.to_dict(),
         }
         return Response(payload)