refactor(backfill): centralize the reserve-yield guard and drop the per-page connection hold

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.

backend/app/quota.py

@@ -118,6 +118,20 @@ def can_spend(db: Session, units: int) -> bool:
     return remaining_today(db) >= units
 
 
+def backfill_reserve_reached() -> bool:
+    """True when today's remaining budget is at or below the backfill reserve — the shared yield
+    condition for every backfill/explore path (recent, deep, immediate full-history, explore, and
+    rolling revalidation).
+
+    Reads in its OWN short-lived session (like record_usage / log_action), for two reasons: it
+    replaces the remaining/reserve comparison that was open-coded at every call site, and — critically
+    for the per-page guards in paging loops — a guard must not hold the caller's transaction open
+    across the network fetch that follows it. Quota is committed independently of any caller, so a
+    fresh session reads the same numbers."""
+    with SessionLocal() as qs:
+        return remaining_today(qs) <= sysconfig.get_int(qs, "backfill_quota_reserve")
+
+
 def log_action(user_id: int, action: str) -> None:
     """Record a zero-cost action event so per-user daily caps still count it.