feat(reader): a Google Takeout export becomes watch state and Watch later

The YouTube API reads neither a user's watch history nor their Watch later list, so the Takeout
export is the only way in. `POST /api/takeout` takes one file of it (watch-history.json or
Watch later-videos.csv, told apart by content), keeps its videos in `takeout_items` and queues the
user's `takeout_import` run, whose outcome reaches their inbox.

The run marks watched the known videos the Shorts probe has cleared, at the Takeout time; a Short
is not marked, and a state the user set here (watched, hidden, or changed after that time) wins, by
the upsert's own condition. A video the catalog does not know yet waits with no outcome. Watch later
videos are fetched if unknown and appended oldest first.

An ad-hoc run can now ask for a completion notice (`run_ad_hoc(..., notify=True)`).

frontend/src/i18n/locales/en/errors.json

@@ -249,6 +249,8 @@
     "api_token_preset_grants": "The Hub token gets its folder by itself — leave the folders out",
     "api_token_no_grants": "Choose at least one folder the token may use",
     "api_token_name_required": "Give the token a name",
-    "api_token_limit": "You already have {{limit}} tokens — revoke one first"
+    "api_token_limit": "You already have {{limit}} tokens — revoke one first",
+    "takeout_unreadable": "This isn't a Takeout file the import reads. Upload watch-history.json (JSON format) or Watch later-videos.csv.",
+    "takeout_no_videos": "This file has no videos to import"
   }
 }