feat(scheduler): a channel's "get full history" backfill is a recorded run, and a row knows its last three runs

The one-channel deep backfill ran as a FastAPI BackgroundTask: what it fetched, what it spent and
how it failed reached only the log. It is now a one-shot run on the scheduler (`channel_backfill`,
trigger ad_hoc, actor = the user, target = the channel) with its quota tagged to the run. Its
per-channel exclusion with the scheduled backfill and the two-run cap are unchanged, and a run
they turn away says why in its result instead of returning nothing. A failure is no longer
swallowed: the run records it.

The scheduler snapshot carries each job's last three runs (one windowed query), so the page can
show an interrupted or skipped run instead of the latest outcome alone.

backend/app/routes/channels.py

@@ -3,15 +3,16 @@ hidden, tags) and per-channel sync state. Local reads/writes plus YouTube-side w
 (subscribe / unsubscribe) that require the optional write scope."""
 
 from datetime import datetime, timezone
+from functools import partial
 
-from fastapi import APIRouter, BackgroundTasks, Depends
+from fastapi import APIRouter, Depends
 from pydantic import BaseModel
 from sqlalchemy import Select, func, select, update
 from sqlalchemy.orm import Session
 
 from app.collation import collated, sorted_words
 from app.mailtext import lang_for_user
-from app import logs, quota
+from app import logs, quota, scheduler
 from app.auth import admin_user, current_user, require_write_scope
 from app.db import get_db
 from app.models import (
@@ -481,7 +482,6 @@ class ChannelUpdateIn(BaseModel):
 def update_channel(
     channel_id: str,
     body: ChannelUpdateIn,
-    background: BackgroundTasks,
     user: User = Depends(current_user),
     db: Session = Depends(get_db),
 ) -> dict:
@@ -508,12 +508,17 @@ def update_channel(
         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).
+    # …and kick the FULL deep backfill for this one channel immediately, as a recorded run of its own,
+    # 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, user.id)
+        scheduler.run_ad_hoc(
+            scheduler.CHANNEL_BACKFILL,
+            partial(backfill_channel_now, channel_id, user.id),
+            actor_id=user.id,
+            target=channel_id,
+        )
 
     return {
         "ok": True,