fix(plex): a name this session cannot write fails at once, and the layout has one spelling

Round 6 turned `artifact_may_arrive` into a positive rule, but keyed it to the HLS vocabulary in
general rather than to what THIS session produces: the playlist regex accepted master/index/stream_*
for every session and the `seq == 0` branch accepted any variant token unconditionally. The two argv
branches are disjoint — a single-audio session is given `seg_%d.ts` + `index.m3u8` and no `%v` at
all, so `master.m3u8`, `stream_*.m3u8` and every `seg_<variant>_<n>.ts` are names it can never write,
and vice versa — so each of those was read as "an HLS name, so wait and see" and held a threadpool
worker, plus its pooled DB connection, for the full timeout. That is verbatim the round-5 outage the
route's own comment claims it prevents.

Measured through the real route against a live single-audio session, before: master.m3u8 404 after
30.0s, stream_q7z.m3u8 404 after 30.0s, seg_a0_0.ts / seg_zzzz_0.ts / seg_0_0.ts 404 after 20.1s
each. After, on the running localdev API: 404 in 44-316ms for all of them, while index.m3u8,
seg_0.ts and seg_1.ts still answer 200 — nothing is over-refused.

The session now carries the rendition names its ffmpeg was told to write, and `entry` is derived from
them instead of passed. And "one place" now means the argv BUILDS its patterns from the same
functions the parser reads back (`segment_name("%d", "%v")`, `variant_playlist_name("%v")`): the
layout was still spelled out independently in six places, with `_VIDEO_VARIANT` read by no consumer,
which is the drift that caused round 6's HIGH. A test asserts the argv's own patterns are what those
functions produce, and that every name they expand to is one the guard accepts.

The load-bearing MEASURED comment cited ffmpeg 8.1.1 — the Windows host's. The binary that writes
these files is the API container's 7.1.5; re-measured there, the conclusion holds.

backend/app/routes/plex/playback.py

@@ -673,12 +673,14 @@ def stream_hls_file(
     if s is None or s.id != session_id:
         raise HTTPException(status_code=404, detail="No active playback session")
     path = s.dir / filename
-    # Only a name this session can actually produce is worth waiting for: the playlists, and a
-    # segment whose predecessor is already on disk (ffmpeg writes them in order, so if it is not
-    # there this one is not next). Answering now instead of waiting it out is what keeps a seek past
-    # the produced region — or a deliberate flood of hopeless names — from parking threadpool
+    # Only a name THIS session can actually produce is worth waiting for: its own playlists, and a
+    # segment of one of its own renditions whose predecessor is already on disk (ffmpeg writes them
+    # in order, so if it is not there this one is not next). The session is passed, not just its
+    # directory, because the single- and multi-audio argv branches spell their names differently and
+    # neither can produce the other's. Answering now instead of waiting it out is what keeps a seek
+    # past the produced region — or a deliberate flood of hopeless names — from parking threadpool
     # workers and their DB connections (see `artifact_may_arrive`).
-    if not plex_stream.artifact_may_arrive(s.dir, filename):
+    if not plex_stream.artifact_may_arrive(s, filename):
         raise HTTPException(status_code=404, detail="Not available yet")
     timeout = 30 if ext == ".m3u8" else 20
     # `proc` so a session whose ffmpeg has already exited answers at once instead of polling out the