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_scheduler.py

@@ -342,6 +342,29 @@ def test_an_ad_hoc_run_is_the_users_run_of_its_target_with_its_spend_and_failure
     assert (failed.status, failed.error) == ("error", "quota exceeded")
 
 
+def _ad_hoc_row(db, job: str) -> dict:
+    rows = scheduler.scheduler_snapshot(db)["ad_hoc"]
+    assert [r["id"] for r in rows] == list(scheduler.AD_HOC_JOBS)  # every one has its row
+    return next(r for r in rows if r["id"] == job)
+
+
+def test_a_takeout_import_has_its_row_named_by_the_importing_user(db, make_user):
+    user = make_user("importer@example.com")
+    user.display_name = "Ida Importer"
+    db.commit()
+    assert _ad_hoc_row(db, "takeout_import")["recent"] == []
+    scheduler._ad_hoc("takeout_import", lambda: {"watched": 2}, user.id, f"user:{user.id}")
+
+    row = _ad_hoc_row(db, "takeout_import")
+    assert (row["running"], row["target"], row["target_title"], row["status"]) == (
+        False,
+        f"user:{user.id}",
+        "Ida Importer",
+        "ok",
+    )
+    assert _ad_hoc_row(db, "channel_backfill")["recent"] == []
+
+
 def test_the_page_shows_the_ad_hoc_runs_beside_the_jobs_with_their_channel(db, make_user):
     user = make_user("shown@example.com")
     db.add(Channel(id="UC_shown", title="BBC Earth"))
@@ -349,7 +372,7 @@ def test_the_page_shows_the_ad_hoc_runs_beside_the_jobs_with_their_channel(db, m
     scheduler._ad_hoc("channel_backfill", lambda: {"skipped": "at capacity"}, user.id, "UC_none")
     scheduler._ad_hoc("channel_backfill", lambda: {"videos_added": 69}, user.id, "UC_shown")
 
-    (row,) = scheduler.scheduler_snapshot(db)["ad_hoc"]
+    row = _ad_hoc_row(db, "channel_backfill")
     assert (row["id"], row["running"], row["target"], row["target_title"]) == (
         "channel_backfill",
         False,
@@ -367,12 +390,12 @@ def test_the_page_shows_the_ad_hoc_runs_beside_the_jobs_with_their_channel(db, m
     seen = []
     scheduler._ad_hoc(
         "channel_backfill",
-        lambda: seen.append(scheduler.scheduler_snapshot(db)["ad_hoc"][0]["running"]) or {},
+        lambda: seen.append(_ad_hoc_row(db, "channel_backfill")["running"]) or {},
         user.id,
         "UC_shown",
     )
     assert seen == [True]
-    assert scheduler.scheduler_snapshot(db)["ad_hoc"][0]["running"] is False
+    assert _ad_hoc_row(db, "channel_backfill")["running"] is False
 
 
 def _finished(db, job) -> list[JobRun]:
@@ -397,6 +420,9 @@ def test_a_second_ad_hoc_request_before_the_first_starts_is_that_same_run(
         "channel_backfill@UC_x",
         "channel_backfill@UC_y",
     ]
+    # Queued, so its target's newest run row is not this run's yet: a page watching it can tell.
+    assert scheduler.ad_hoc_queued(scheduler.CHANNEL_BACKFILL, "UC_x")
+    assert not scheduler.ad_hoc_queued(scheduler.CHANNEL_BACKFILL, "UC_z")
     live_scheduler.remove_all_jobs()  # never run: a run finishing after the test would land in the next
 
 
@@ -475,7 +501,7 @@ def test_a_run_whose_outcome_cannot_be_written_is_not_called_failed(db, make_use
     scheduler._ad_hoc("channel_backfill", lambda: {"videos_added": 1}, user.id, "UC_blip")
     assert calls == ["ok"]
     # Its row is still `running`, but nothing here is: the page does not call it going.
-    (row,) = scheduler.scheduler_snapshot(db)["ad_hoc"]
+    row = _ad_hoc_row(db, "channel_backfill")
     assert (row["running"], row["status"], row["recent"][0]["status"]) == (
         False,
         "interrupted",
@@ -529,7 +555,7 @@ def test_an_ad_hoc_run_still_going_shows_although_a_newer_one_ended(db, make_use
         return {}
 
     scheduler._ad_hoc("channel_backfill", long_one, user.id, "UC_long")
-    (row,) = rows
+    (row,) = [r for r in rows if r["id"] == "channel_backfill"]
     assert (row["running"], row["target"], row["target_title"]) == (True, "UC_long", "Long one")
     # The outcome beside it is the long one's (none yet), never the short one's skip.
     assert (row["status"], row["result"], row["last_error"]) == (None, None, None)