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/tests/test_channel_backfill.py

@@ -227,7 +227,7 @@ class TestDeepBackfillPaging:
         return ch
 
     def test_cursor_is_committed_each_page_and_a_later_run_resumes(self, db, monkeypatch):
-        monkeypatch.setattr(videos.quota, "remaining_today", lambda _db: 10_000)
+        monkeypatch.setattr(videos.quota, "backfill_reserve_reached", lambda: False)  # budget ok
         ch = self._channel(db)
         yt = _PagedYt(
             {
@@ -258,11 +258,11 @@ class TestDeepBackfillPaging:
         assert ch.backfill_cursor is None
 
     def test_yields_when_the_shared_budget_hits_the_reserve(self, db, monkeypatch):
-        # Budget is above the 2000 reserve for the first page, then drops to the reserve — the guard
-        # is checked BEFORE each page, so the run fetches exactly one page and then yields, leaving
-        # the tail for the paced scheduler (cursor parked, backfill_done False).
-        budget = iter([10_000, 2_000, 2_000])
-        monkeypatch.setattr(videos.quota, "remaining_today", lambda _db: next(budget))
+        # Budget is fine for the first page, then hits the reserve — the guard is checked BEFORE each
+        # page, so the run fetches exactly one page and then yields, leaving the tail for the paced
+        # scheduler (cursor parked, backfill_done False).
+        reached = iter([False, True])
+        monkeypatch.setattr(videos.quota, "backfill_reserve_reached", lambda: next(reached))
         ch = self._channel(db)
         yt = _PagedYt(
             {