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/routes/channels.py

@@ -4,7 +4,7 @@ hidden, tags) and per-channel sync state. Read + local-write only — YouTube-si
 import logging
 from datetime import datetime, timezone
 
-from fastapi import APIRouter, Depends, HTTPException
+from fastapi import APIRouter, BackgroundTasks, Depends, HTTPException
 from pydantic import BaseModel
 from sqlalchemy import func, select, update
 from sqlalchemy.orm import Session
@@ -28,7 +28,7 @@ from app.models import (
 from app.routes._common import owned_or_404
 from app.routes.admin import admin_user
 from app.sync.explore import explore_ingest_page
-from app.sync.runner import run_recent_backfill
+from app.sync.runner import backfill_channel_now, run_recent_backfill
 from app.sync.subscriptions import apply_channel_details
 from app.utils import iso
 from app.youtube.client import YouTubeClient, YouTubeError
@@ -449,6 +449,7 @@ class ChannelUpdateIn(BaseModel):
 def update_channel(
     channel_id: str,
     body: ChannelUpdateIn,
+    background: BackgroundTasks,
     user: User = Depends(current_user),
     db: Session = Depends(get_db),
 ) -> dict:
@@ -473,12 +474,18 @@ def update_channel(
 
     # Opting a channel into full history should give an immediate feed: if we haven't even
     # fetched its recent uploads yet, do that one channel now (cheap) instead of waiting for
-    # the scheduler. Deep paging still follows on the scheduler's next run (recent-then-deep).
-    # (deep_turned_on is only ever True for a non-demo user — see the guard above.)
+    # the scheduler. (deep_turned_on is only ever True for a non-demo user — see the guard above.)
     if deep_turned_on and channel is not None and channel.recent_synced_at is None:
         with quota.attribute(user.id, quota.QuotaAction.VIDEOS_BACKFILL_RECENT):
             run_recent_backfill(db, [channel], max_channels=1)
 
+    # …and kick the FULL deep backfill for this one channel immediately in the background, so the
+    # whole catalogue lands in seconds instead of over the scheduler's paced 10-minute ticks. The
+    # request returns right away; the channel page's live poll shows the count climb. The scheduler
+    # still finishes a very large channel's tail on its normal runs (deep_requested stays set).
+    if deep_turned_on:
+        background.add_task(backfill_channel_now, channel_id)
+
     return {
         "id": channel_id,
         "priority": sub.priority,