fix(api): progress checkpoints past the int4 column are a 422, and idle HLS sessions are reaped on a timer

Deep-review round 3 of bug hunt 2026-09.

- The YouTube and Plex progress bodies are bounded by POSITION_MAX_S (the int4 position columns): a
  finite position of 2^31 or more was a DataError, i.e. a 500 — the sibling of round 2's non-finite
  fix.
- plex/stream.py promised "a periodic reaper", but reap_idle ran only inside the NEXT session
  request: after the last playback an abandoned session (its ffmpeg still working, its directory a
  whole segmented copy of the film) lived until someone pressed play again. A lifespan task reaps
  every 60 s.
- Tests for what round 2 left untested: the 422's body keeps the list shape the UI localizes, and
  stream_session refuses non-finite start/aoff.

backend/app/models.py

@@ -24,6 +24,11 @@ from app.db import Base
 from app.utils import canonical_email
 
 
+# The resume-position columns (VideoState / PlexState.position_seconds) are int4. The checkpoint
+# routes bound their bodies by this, so an out-of-range position is a 422 rather than a 500.
+POSITION_MAX_S = 2**31 - 1
+
+
 class TimestampMixin:
     """Adds a server-defaulted `created_at`. Models that index created_at or order it
     differently (QuotaEvent, Notification, Message) keep their own column on purpose."""
@@ -423,6 +428,7 @@ class VideoState(Base, UpdatedAtMixin):
     # status: a partially-watched video keeps status "new" but a non-zero position, so
     # it can render a progress bar and match the "in progress" feed filter. 0 = not
     # started / finished (trivially-early and near-finished positions are not stored).
+    # int4: a checkpoint body must stay under POSITION_MAX_S, or the flush is a DataError (a 500).
     position_seconds: Mapped[int] = mapped_column(
         Integer, default=0, server_default="0", nullable=False
     )