feat(channel): live-refresh the channel page while full-history backfills

The channel detail page now polls (via useLiveQuery) while THIS user's
full-history backfill is running (deep_requested && !backfill_done), so
the stored count and the N/S/L breakdown climb on their own as the
scheduler adds videos — no manual F5 — and it stops polling once the
back-catalogue is in. Widen useLiveQuery's interval callback to allow
returning `false` (stop) so "poll while active, stop when done" is a
first-class, reusable cadence.

frontend/src/lib/useLiveQuery.ts

@@ -9,10 +9,13 @@ export function useLiveQuery<T>(
   key: QueryKey,
   queryFn: () => Promise<T>,
   // intervalMs may be a function of the latest data, so the poll cadence can adapt to it
-  // (e.g. poll faster while a job is running). react-query re-evaluates it after each fetch
-  // against fresh data — the right place for this, vs. a React state toggle that can lag or
-  // stall the live updates.
-  opts: { intervalMs?: number | ((data: T | undefined) => number); enabled?: boolean } = {},
+  // (e.g. poll faster while a job is running, and return `false` to STOP once it's done).
+  // react-query re-evaluates it after each fetch against fresh data — the right place for this,
+  // vs. a React state toggle that can lag or stall the live updates.
+  opts: {
+    intervalMs?: number | false | ((data: T | undefined) => number | false);
+    enabled?: boolean;
+  } = {},
 ) {
   const { intervalMs = 4000, enabled = true } = opts;
   return useQuery({