fix(reader): the Takeout import keeps a later upload, an un-mark and its memory bound

- An ad-hoc request made while its target's run is under way is the run after it: the scheduler
  dropped a second fire of a running id, so a second Takeout file was never imported and its key
  stayed queued until a restart.
- Un-marking or resetting a video is remembered (video_unmarks): an import of an older YouTube view
  no longer marks it watched again; un-marks take the user's Takeout lock.
- One user's failure in the Shorts job's Takeout pass is theirs: the job keeps its own result, the
  others are settled, and a failed lookup spends none of the daily share.
- The upload reads the user's id before giving the read connection back.
- A watched row waits on a discovery video (search or explore) until the catalog keeps it, so a
  mark never pins it against the cleanup jobs.
- The history is decoded a window at a time, a Watch later file over 2 MiB is refused unread, and
  two uploads at once across all users (takeout_server_busy).
- A Watch later list goes in as one presence read and one insert.

backend/tests/conftest.py

@@ -327,3 +327,15 @@ def _reset_log_erasures():
     from app import logs
 
     logs._erased.clear()
+
+
+@pytest.fixture(autouse=True)
+def _reset_ad_hoc_queue():
+    """The scheduler's ad-hoc queue is module-level too: a request a test left pending would read as
+    a later test's user's import still queued (ids restart from 1)."""
+    from app import scheduler
+
+    with scheduler._queued_guard:
+        scheduler._queued.clear()
+        scheduler._pending.clear()
+        scheduler._running.clear()