feat(channels): admin reset / re-backfill endpoint

POST /api/channels/{id}/reset-backfill (admin-only): clear the channel's backfill
markers, re-opt into deep backfill and re-run its recent pull now — a re-fetch-from-
scratch trigger regardless of current sync state. Idempotent (videos upsert by id).

backend/app/routes/channels.py

@@ -9,6 +9,7 @@ from app import quota
 from app.auth import current_user, has_write_scope
 from app.db import get_db
 from app.models import Channel, ChannelTag, Subscription, Tag, User, Video
+from app.routes.admin import admin_user
 from app.sync.runner import run_recent_backfill
 from app.youtube.client import YouTubeClient, YouTubeError
 
@@ -134,6 +135,30 @@ def update_channel(
     }
 
 
+@router.post("/{channel_id}/reset-backfill")
+def reset_backfill(
+    channel_id: str,
+    user: User = Depends(admin_user),
+    db: Session = Depends(get_db),
+) -> dict:
+    """Admin-only reset: re-fetch this channel from scratch regardless of its current sync
+    state (a "reset" trigger). Clears the channel's backfill markers, opts it back into deep
+    backfill, and re-runs its recent pull immediately; the deep scheduler re-pages the full
+    back-catalog on its next run. Idempotent — videos upsert by id, so nothing duplicates."""
+    channel = db.get(Channel, channel_id)
+    if channel is None:
+        raise HTTPException(status_code=404, detail="Unknown channel")
+    sub = _user_subscription(db, user, channel_id)
+    channel.backfill_done = False
+    channel.backfill_cursor = None
+    channel.recent_synced_at = None
+    sub.deep_requested = True
+    db.commit()
+    with quota.attribute(user.id, "backfill_recent"):
+        run_recent_backfill(db, [channel], max_channels=1)
+    return {"id": channel_id, "reset": True}
+
+
 @router.delete("/{channel_id}/subscription")
 def unsubscribe(
     channel_id: str,

frontend/src/lib/api.ts

@@ -381,6 +381,8 @@ export const api = {
     id: string,
     patch: { priority?: number; hidden?: boolean; deep_requested?: boolean }
   ) => req(`/api/channels/${id}`, { method: "PATCH", body: JSON.stringify(patch) }),
+  resetChannelBackfill: (id: string) =>
+    req(`/api/channels/${id}/reset-backfill`, { method: "POST" }),
   deepAll: (on = true) =>
     req(`/api/sync/deep-all?on=${on}`, { method: "POST" }),
   attachChannelTag: (id: string, tagId: number) =>