fix(reader): the Takeout import holds up under abuse, races and a run it cannot see

- Uploads: one at a time per user and ten an hour, each refused only after its body arrived;
  the history read an entry at a time (32 MiB cap), a Watch later list capped at YouTube's 5000,
  a user's kept videos at 200k; any unreadable shape is a 422, not a 500.
- Quota: Watch later lookups stop at the backfill reserve and at a per-user daily share; what is
  not asked about waits, known videos still go in, and the shorts job retries the rest.
- Staging and settling share one per-user advisory lock (no deadlock); a run settles only the
  rows it read, so a row staged meanwhile is never called gone.
- A failed run's notice names a translated failure, never the upstream's text.
- The status says when an import is queued; the page watches it to its end even when left, then
  reads the feed, its counts and the playlists again.
- Settings: one live region, the zone kept (unavailable) while a run goes, focus handed back,
  a missed drop kept from the browser, an oversize file refused before it is sent.
- Scheduler and run lists show Takeout runs by the importing user.

frontend/src/modules/settings/takeoutWatch.ts

@@ -0,0 +1,57 @@
+import type { QueryClient } from "@tanstack/react-query";
+import { api, type TakeoutStatus } from "../../lib/api";
+import { qk } from "../../lib/queryKeys";
+
+// A Takeout import, watched to its end: its status is asked again until no run is queued or going,
+// then the lists it changed are read afresh. It lives outside the section on purpose: the user may
+// leave Settings while the import runs, and the feed they go to must not keep showing as new what
+// the import has just marked watched.
+
+const POLL_MS = 1500;
+
+export const importBusy = (s: TakeoutStatus | undefined): boolean =>
+  !!s && (s.queued || s.run?.status === "running");
+
+let watching = false;
+
+const pause = () => new Promise<void>((done) => setTimeout(done, POLL_MS));
+
+async function read(qc: QueryClient): Promise<TakeoutStatus | undefined> {
+  try {
+    return await qc.fetchQuery({
+      queryKey: qk.takeout(),
+      queryFn: api.takeoutStatus,
+      staleTime: 0,
+    });
+  } catch {
+    // A read that failed is the section's to say (its query holds the error); the watch asks again.
+    return qc.getQueryData<TakeoutStatus>(qk.takeout());
+  }
+}
+
+/** Mark stale everything an import may have changed: the feed and its counts, Watch later, and the
+ *  playlists that show its size. */
+export function invalidateImported(qc: QueryClient): void {
+  for (const queryKey of [
+    qk.feed(),
+    qk.feedSummary(),
+    qk.feedYears(),
+    qk.playlist(),
+    qk.playlists(),
+  ])
+    void qc.invalidateQueries({ queryKey });
+}
+
+/** Watch the user's import until it is done (once: a second call while it watches is that watch),
+ *  then mark stale what it changed. */
+export async function watchTakeout(qc: QueryClient): Promise<void> {
+  if (watching) return;
+  watching = true;
+  let status = qc.getQueryData<TakeoutStatus>(qk.takeout());
+  while (importBusy(status)) {
+    await pause();
+    status = await read(qc);
+  }
+  watching = false;
+  invalidateImported(qc);
+}