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/maintenance.py

@@ -25,7 +25,7 @@ from datetime import timedelta
 from sqlalchemy import select
 from sqlalchemy.orm import Session
 
-from app import progress, quota, sysconfig
+from app import progress, quota
 from app.config import settings
 from app.models import Playlist, PlaylistItem, Video
 from app.notifications import create_notification
@@ -188,7 +188,7 @@ def _revalidate_rolling(db: Session, yt: YouTubeClient) -> dict:
     now = _now()
     # Process in videos.list-sized pages so we commit incrementally and stop on low quota.
     while checked < batch:
-        if quota.remaining_today(db) <= sysconfig.get_int(db, "backfill_quota_reserve"):
+        if quota.backfill_reserve_reached():
             break
         page = (
             db.execute(
@@ -228,7 +228,7 @@ def run_maintenance(db: Session) -> dict:
         phase1 = _recheck_flagged(db, yt)
         phase2 = (
             _revalidate_rolling(db, yt)
-            if quota.remaining_today(db) > sysconfig.get_int(db, "backfill_quota_reserve")
+            if not quota.backfill_reserve_reached()
             else {"checked": 0, "flagged": 0, "skipped": "low quota"}
         )
     return {

backend/app/sync/runner.py

@@ -183,7 +183,7 @@ def run_recent_backfill(
     videos_added = 0
     with YouTubeClient(db, user) as yt:
         for channel in channels:
-            if quota.remaining_today(db) <= sysconfig.get_int(db, "backfill_quota_reserve"):
+            if quota.backfill_reserve_reached():
                 break
             try:
                 videos_added += backfill_channel_recent(db, yt, channel)
@@ -229,7 +229,7 @@ def run_deep_backfill(db: Session, max_channels: int = 10, max_pages: int = 5) -
     videos_added = 0
     with YouTubeClient(db, user) as yt:
         for channel in channels:
-            if quota.remaining_today(db) <= sysconfig.get_int(db, "backfill_quota_reserve"):
+            if quota.backfill_reserve_reached():
                 break
             # Claim the channel (or skip it if an interactive backfill already owns it) so the two
             # never advance the same cursor concurrently — symmetric with backfill_channel_now.
@@ -294,7 +294,7 @@ def backfill_channel_now(channel_id: str, user_id: int, max_pages: int = 400) ->
             channel = db.get(Channel, channel_id)
             if channel is None or channel.backfill_done or not channel.uploads_playlist_id:
                 return
-            if quota.remaining_today(db) <= sysconfig.get_int(db, "backfill_quota_reserve"):
+            if quota.backfill_reserve_reached():
                 return
             try:
                 # Attribute the spend to the requesting user under the full-backfill action, so it

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: