Cancel Subscription

Cancel the user's subscription.

Steps:

1. Generate a final Stripe invoice for any unbilled completed optimizations
2. Mark users.is_active = FALSE, users.cancelled_at = NOW()
3. Mark youtube_channels.is_active = FALSE for the user's channels
4. Mark videos.optimization_paused = TRUE for the user's videos
5. Mark in-flight (status='measuring') video_optimizations as billable=FALSE
6. Send cancellation notification

Already-enqueued Cloud Tasks are NOT walked or deleted; the gate added to optimization_queue.process_single_optimization will no-op them.