feat(channel): backfill a channel's full history immediately on request

Clicking "get full history" now kicks an immediate background deep
backfill of that one channel (backfill_channel_now: own session, up to
~3000 videos in one pass, quota-guarded) instead of waiting for the
scheduler's paced 10-minute ticks (5 pages/channel/tick). The request
returns instantly and the channel page's live poll shows the count fill
in seconds. A very large channel's tail still falls to the scheduler
(deep_requested stays set).

backend/app/sync/runner.py

@@ -13,6 +13,7 @@ from sqlalchemy.orm import Session
 from app import progress, quota, sysconfig
 
 log = logging.getLogger("siftlode.sync")
+from app.db import SessionLocal
 from app.models import Channel, OAuthToken, Subscription, User
 from app.sync.subscriptions import import_subscriptions
 from app.sync.videos import (
@@ -244,6 +245,30 @@ def run_deep_backfill(db: Session, max_channels: int = 10, max_pages: int = 5) -
     }
 
 
+def backfill_channel_now(channel_id: str, max_pages: int = 60) -> None:
+    """User-initiated "get full history": deep-backfill ONE channel to completion right now, in a
+    background task with its OWN session, so the full catalogue lands in seconds instead of over the
+    paced scheduler's 10-minute ticks (max_pages=5/channel/tick). Quota-guarded; a very large
+    channel's tail (beyond ~max_pages*50 videos) is finished by the scheduler on its normal runs,
+    since deep_requested stays set. Best-effort — logs and swallows failures."""
+    with SessionLocal() as db:
+        user = get_service_user(db)
+        if user is None:
+            return
+        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"):
+            return
+        try:
+            with YouTubeClient(db, user) as yt:
+                backfill_channel_deep(db, yt, channel, max_pages)
+            reconcile_full_history(db)
+        except Exception:
+            db.rollback()
+            log.exception("Immediate backfill failed for channel %s", channel_id)
+
+
 def estimate_deep_backfill(db: Session, channels: list[Channel]) -> dict:
     """Rough ETA to finish full-history backfill of `channels`. Quota-bound: total
     estimated API units / the daily budget left for backfill. Deliberately approximate —