fix(backfill): finish large channels in one immediate full-history run

The user-triggered "get full history" (backfill_channel_now) capped at
max_pages=60 = 3000 videos. A channel just over that (BBC Earth: 3403
uploads = 69 pages) stopped one page short, so its tail dribbled in over
the scheduler's paced 10-minute ticks (250 videos/tick) and the button
spinner looked stuck for ~18 minutes with no visible progress.

- Raise the immediate cap to max_pages=400 (20 000 videos) so any
  realistic channel completes in the single background run.
- Make a high cap safe: backfill_channel_deep now re-checks the shared
  quota before each page and yields at the backfill reserve (previously
  quota was checked once by the caller before the whole run), and commits
  the advanced cursor after every page so a restart resumes instead of
  re-fetching — and re-charging quota for — pages already stored.
- Channel page: while loading, the button shows the live stored/total
  count and a tooltip explaining the tail finishes in the background, so a
  very large channel's paced remainder reads as progress, not a hang.

Adds deep-backfill paging tests (per-page cursor commit + resume, and the
per-page quota yield).

backend/app/sync/videos.py

@@ -10,7 +10,7 @@ from sqlalchemy import and_, func, or_, select, update
 from sqlalchemy.dialects.postgresql import insert as pg_insert
 from sqlalchemy.orm import Session
 
-from app import progress, sysconfig
+from app import progress, quota, sysconfig
 from app.models import LIVE_OR_UPCOMING, Channel, Video
 from app.titles import normalize_title
 from app.youtube.client import YouTubeClient, best_thumbnail
@@ -156,12 +156,22 @@ def backfill_channel_recent(db: Session, yt: YouTubeClient, channel: Channel) ->
 def backfill_channel_deep(
     db: Session, yt: YouTubeClient, channel: Channel, max_pages: int = 5
 ) -> int:
-    """Continue paging older uploads from the stored cursor (bounded per run)."""
+    """Continue paging older uploads from the stored cursor (bounded per run).
+
+    Commits the advanced cursor after every page (not just at the end) so progress
+    is durable: a restart resumes from the last stored page instead of re-fetching —
+    and re-charging quota for — pages already ingested. Re-checks the shared budget
+    before each page and yields once it hits the backfill reserve, so a single large
+    channel can't drain the day's quota in one run even with a high max_pages; the
+    paced scheduler finishes the tail on later ticks (deep_requested stays set)."""
     if channel.backfill_done or not channel.uploads_playlist_id:
         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:
+            break
         data = yt.get_playlist_items(channel.uploads_playlist_id, page_token)
         rows = [
             s
@@ -175,9 +185,10 @@ def backfill_channel_deep(
         channel.backfill_cursor = page_token
         if not page_token:
             channel.backfill_done = True
+        db.add(channel)
+        db.commit()
+        if not page_token:
             break
-    db.add(channel)
-    db.commit()
     return total