A freshly created subscription has no gallery-dl archive yet, so the
first poll in tick mode would walk forever — exit:20 doesn't trip
until 20 contiguous archive-hits, and there are none. The wall-clock
cap kicks in mid-walk, and the partial-success classifier (plan #544)
gracefully labels it status=ok, but the operator still wonders why
their new subscription isn't grabbing the back-catalog.
Pre-arm `backfill_runs_remaining = 3` on create() when `enabled=True`.
Same default as the manual "Deep scan" button, same auto-decrement
and auto-reset rules — once the queue drains (clean exit + zero new
files) or the budget runs out, tick mode resumes naturally.
Disabled sources (sidecar synthetics with `url='sidecar:<platform>:<slug>'`
that arrive disabled = False, or operator-added sources that start
disabled deliberately) skip the pre-arm — they're never polled, no
budget to waste.
bvandeusen
2aa2002f22