Send Tracking Credit Notice

Run the out-of-credits notice sweep now.

The scheduler runs this on its own every TRACKING_CREDIT_NOTICE_SWEEP_HOURS; this endpoint exists so an operator can reach users who are already stalled without waiting for the next sweep.

force=true ignores the per-user cooldown — for the first run after deploy, or to re-send after fixing a bad address. Use it knowingly: it will mail everyone currently paused.