fix(sync): a channel with no uploads backfills as done, not as a failure

YouTube answers the uploads list of a channel with no uploads as
playlistNotFound. The recent and deep backfills raised on it, so the channel
was never stamped and was picked again, charged a quota unit and logged an
ERROR on every run. Both now read it as an empty last page, as explore already
does; any other failure still raises.

backend/app/sync/videos.py

@@ -11,12 +11,14 @@ 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, quota, sysconfig
+from app import logs, 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
+from app.youtube.client import YouTubeClient, YouTubeError, best_thumbnail, is_playlist_gone
 from app.youtube.shorts import make_client, probe_is_short
 
+log = logs.get_logger(__name__)
+
 _DURATION_RE = re.compile(r"P(?:(\d+)D)?T(?:(\d+)H)?(?:(\d+)M)?(?:(\d+)S)?", re.IGNORECASE)
 
 
@@ -104,6 +106,20 @@ def stub_from_playlist_item(item: dict, channel_id: str) -> dict | None:
     }
 
 
+def _uploads_page(yt: YouTubeClient, channel_id: str, uploads: str, page_token: str | None) -> dict:
+    """One page of the channel's uploads. YouTube answers the uploads list of a channel with no
+    uploads at all as not found: an empty back-catalogue, so an empty last page — not a failure
+    that left `recent_synced_at` unstamped and charged the channel again on every run (E32 review).
+    Any other failure raises."""
+    try:
+        return yt.get_playlist_items(uploads, page_token)
+    except YouTubeError as exc:
+        if not is_playlist_gone(exc):
+            raise
+        log.info("backfill channel=%s has no uploads playlist", channel_id)
+        return {}
+
+
 def backfill_channel_recent(db: Session, yt: YouTubeClient, channel: Channel) -> int:
     """First pass: newest videos up to the configured count or age cutoff. Records a
     resume cursor for the later deep backfill."""
@@ -121,7 +137,7 @@ def backfill_channel_recent(db: Session, yt: YouTubeClient, channel: Channel) ->
 
     while True:
         page_start = page_token  # token that fetched THIS page (None = first page)
-        data = yt.get_playlist_items(channel.uploads_playlist_id, page_token)
+        data = _uploads_page(yt, channel.id, channel.uploads_playlist_id, page_token)
         stopped_mid_page = False
         for item in data.get("items", []):
             stub = stub_from_playlist_item(item, channel.id)
@@ -175,7 +191,7 @@ def backfill_channel_deep(
     for _ in range(max_pages):
         if quota.backfill_reserve_reached():
             break
-        data = yt.get_playlist_items(channel.uploads_playlist_id, page_token)
+        data = _uploads_page(yt, channel.id, channel.uploads_playlist_id, page_token)
         rows = [
             s
             for s in (stub_from_playlist_item(it, channel.id) for it in data.get("items", []))