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/sync/videos.py

@@ -168,9 +168,8 @@ def backfill_channel_deep(
         return 0
     page_token = channel.backfill_cursor
     total = 0
-    reserve = sysconfig.get_int(db, "backfill_quota_reserve")
     for _ in range(max_pages):
-        if quota.remaining_today(db) <= reserve:
+        if quota.backfill_reserve_reached():
             break
         data = yt.get_playlist_items(channel.uploads_playlist_id, page_token)
         rows = [
@@ -183,8 +182,7 @@ def backfill_channel_deep(
         total += insert_stubs(db, rows)
         page_token = data.get("nextPageToken")
         channel.backfill_cursor = page_token
-        if not page_token:
-            channel.backfill_done = True
+        channel.backfill_done = not page_token
         db.add(channel)
         db.commit()
         if not page_token: