Addresses the code-review findings on the immediate full-history change:
- Extract quota.backfill_reserve_reached(): the remaining/reserve compare was
open-coded at seven sites (recent/deep/immediate backfill, explore, rolling
revalidation, and the new per-page guard). It reads in its OWN short-lived
session (like record_usage/log_action), so a per-page guard no longer holds
the caller's transaction open across the YouTube fetch that follows — the
400-page immediate run no longer pins a pooled connection idle-in-transaction
through each network round-trip. Migrated all call sites; dropped the now-dead
sysconfig imports in channels.py and maintenance.py.
- Simplify the deep-backfill loop: channel.backfill_done = not page_token (done
is False on entry) plus one break, replacing the doubled `if not page_token`.
- Tests patch the backfill_reserve_reached seam directly instead of mocking
remaining_today internals, removing the fragile dead-value budget iterator.
Full backend suite green; ruff clean.
peter
48285a715b