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).

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) =>