fix(plex): playback sessions per viewer, bounded in total; a fair share of subtitle passes

HLS sessions were keyed by the item, so a second account opening the same
film killed the first one's session; the displaced player reclaimed at its
playhead and evicted the other in turn. The registry is now keyed by
(viewer, rating_key), segment requests resolve by both, and the session
directory carries the viewer (the orphan sweep still recognises the older
shapes).

Only transcodes counted toward a cap, so remuxes — each a whole segmented
copy of the film on the scratch volume — were bounded by the library size.
Three bounds now apply after every start, each dropping the least recently
used: two sessions per account, the transcode cap, and a new admin knob,
plex_max_sessions (default 8), over every session. A superseded session
request returns at once, since it waits on the killed ffmpeg, so an account
cannot park more request workers than it may hold sessions.

The subtitle extractor's full-pass slots were global and minutes long each;
one account opening subtitles on film after film could hold them all. An
account may now hold three of the six, and still gets windows past that.

backend/app/routes/plex/playback.py

@@ -121,7 +121,7 @@ def subtitle_status(
 
     `at` is the viewer's absolute position: the extractor windows around it, so what gets read first
     is the part actually about to be watched."""
-    return subtitles.ensure(rating_key, sub_ord, at)
+    return subtitles.ensure(rating_key, sub_ord, at, viewer=user.id)
 
 
 @router.get("/subtitle/{rating_key}/{sub_ord}")
@@ -362,7 +362,7 @@ def item_progress(
     # finish latch is up, or before the first position is known. Those are the states in which
     # reaping is the right answer anyway, and a viewer who scrubs back into a reaped session gets
     # the same 404 → reclaim the player already handles.
-    plex_stream.touch_session(str(rating_key))
+    plex_stream.touch_session(user.id, str(rating_key))
     position = max(0, int(body.position_seconds))
     duration = int(body.duration_seconds)
     final = body.final
@@ -607,7 +607,7 @@ def stream_session(
         }
     if plex_paths.local_media_path(db, it.file_path) is None:
         raise HTTPException(status_code=404, detail="Media file not found on disk")
-    s = plex_stream.start_session(db, it, start, audio, aoff, multi, transcode=transcode)
+    s = plex_stream.start_session(db, it, user.id, start, audio, aoff, multi, transcode=transcode)
     if s is None:
         raise HTTPException(status_code=404, detail="Media file not found on disk")
     # Report the REAL media start (the keyframe ffmpeg landed on, which hls.js zero-bases to), not the
@@ -677,9 +677,9 @@ def stream_hls_file(
     references its segments/subtitles by relative name, all resolving back here.
 
     The session id in the path is what makes "back here" mean the same session. Sessions are keyed by
-    rating_key alone and a seek-restart or a second viewer of the same item replaces the registered
-    one, while the DIRECTORY is per (key, offset, tag) — so resolving by rating_key handed the old
-    viewer the new session's `seg_12.ts`, produced from a different offset under `-copyts`: wrong
+    (viewer, rating_key) and a seek-restart replaces the viewer's registered one, while the DIRECTORY
+    is per (key, viewer, offset, tag) — so resolving by the key alone handed the old player the new
+    session's `seg_12.ts`, produced from a different offset under `-copyts`: wrong
     bytes with a 200, which the player has no way to notice (deep-review round 5). A request for a
     session that is no longer the registered one is a 404, and the player reclaims at the playhead.
 
@@ -695,7 +695,7 @@ def stream_hls_file(
         or ".." in filename
     ):
         raise HTTPException(status_code=404, detail="Not found")
-    s = plex_stream.current_session(str(rating_key))
+    s = plex_stream.current_session(user.id, str(rating_key))
     if s is None or s.id != session_id:
         raise HTTPException(status_code=404, detail="No active playback session")
     path = s.dir / filename