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.

backend/tests/test_body_limit.py

@@ -23,6 +23,7 @@ from app import bodylimit, debugsession, sysconfig
 from app.config import settings
 from app.models import SystemConfig
 from app.bodylimit import DEFAULT_MAX_BODY_BYTES, BodyLimitMiddleware
+from app.reader import takeout
 from app.routes import diagnostics as diagnostics_routes
 
 SMALL = b"x" * 16
@@ -369,6 +370,9 @@ def test_every_body_reading_site_is_known_and_fits_under_its_ceiling():
     # The upload site's limit is the sysconfig knob, which is bounded BY the edge ceiling rather
     # than being a constant to compare against — that pairing is its own test below.
     assert ("POST", "/api/files/upload") in bodylimit.ROUTE_MAX_BODY_BYTES
+    # A Takeout file is larger than the default: without its ceiling every real history is a 413.
+    assert takeout.MAX_BYTES > DEFAULT_MAX_BODY_BYTES
+    assert bodylimit.ROUTE_MAX_BODY_BYTES[("POST", "/api/takeout")] == takeout.MAX_BYTES
 
 
 def test_release_read_connection_ends_the_transaction_it_was_called_for(db):