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 A superseded session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.

frontend/src/i18n/locales/en/config.json

@@ -193,7 +193,11 @@
     },
     "plex_max_transcodes": {
       "label": "Max concurrent transcodes",
-      "hint": "Cap on simultaneous transcode sessions for non-H.264 files. Direct-play and remux (stream-copy) files aren't limited."
+      "hint": "Cap on simultaneous transcode sessions for non-H.264 files. Remux (stream-copy) sessions count only toward the overall session limit; direct-play files aren't limited."
+    },
+    "plex_max_sessions": {
+      "label": "Max concurrent playback sessions",
+      "hint": "Cap on all simultaneous playback sessions, remux (stream-copy) included — each one writes a full segmented copy of the film to the scratch volume. One account can hold at most two. Direct-play files aren't sessions and aren't limited."
     },
     "plex_transcode_hw": {
       "label": "Hardware transcode",

frontend/src/i18n/locales/hu/config.json

@@ -193,7 +193,11 @@
     },
     "plex_max_transcodes": {
       "label": "Max párhuzamos transcode",
-      "hint": "A nem-H.264 fájlok egyidejű transcode-munkameneteinek felső korlátja. A direktben játszható és a remux (stream-copy) fájlokra nincs korlát."
+      "hint": "A nem-H.264 fájlok egyidejű transcode-munkameneteinek felső korlátja. A remux (stream-copy) munkamenetek csak az összesített munkamenet-korlátba számítanak bele; a direktben játszható fájlokra nincs korlát."
+    },
+    "plex_max_sessions": {
+      "label": "Max párhuzamos lejátszási munkamenet",
+      "hint": "Az összes egyidejű lejátszási munkamenet felső korlátja, a remuxot (stream-copy) is beleértve — mindegyik a film teljes, szegmentált másolatát írja a munkakötetre. Egy fiók legfeljebb kettőt tarthat. A direktben játszható fájlok nem munkamenetek, rájuk nincs korlát."
     },
     "plex_transcode_hw": {
       "label": "Hardveres transcode",