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/components/ui/DropZone.tsx

@@ -1,4 +1,4 @@
-import { useRef, useState, type ReactNode } from "react";
+import { useRef, useState, type ReactNode, type Ref } from "react";
 import clsx from "clsx";
 import { Pressable } from "./Pressable";
 
@@ -8,6 +8,7 @@ import { Pressable } from "./Pressable";
 // children (an icon, a line of text), so a plain boolean flickers — enter and leave are counted
 // and the highlight clears at depth 0. The caller draws the inside, told whether a drag is over it.
 export function DropZone({
+  ref,
   onFiles,
   multiple = false,
   accept,
@@ -17,6 +18,8 @@ export function DropZone({
   className,
   children,
 }: {
+  /** The zone's button, for a caller that moves the focus to it. */
+  ref?: Ref<HTMLButtonElement>;
   /** The files dropped or picked, never empty. One unless `multiple`. */
   onFiles: (files: [File, ...File[]]) => void;
   multiple?: boolean;
@@ -40,6 +43,7 @@ export function DropZone({
   return (
     <>
       <Pressable
+        ref={ref}
         align="center"
         onClick={() => inputRef.current?.click()}
         onDragEnter={(e) => {