fix(player): only the credits that end an item move on to the next one

An item can carry several credits blocks with mid- and post-credit scenes between
them, and Plex flags the one that really ends it. The player left at the first
block. Now the Plex-final block moves on (by the auto-skip or the Skip button), an
earlier block is only skipped to its end by the button, and with no final flag the
last block ends the item by the button alone.

backend/app/routes/plex/detail.py

@@ -404,6 +404,10 @@ def item_detail(
                         "type": m.get("type"),
                         "start_s": round((m.get("startTimeOffset") or 0) / 1000, 1),
                         "end_s": round((m.get("endTimeOffset") or 0) / 1000, 1),
+                        # An item can carry several credits blocks with scenes between them; Plex
+                        # flags the one that really ends it. Which credits may leave the item is the
+                        # player's rule (`lib/plexMarkers`), so the flag goes over as Plex sent it.
+                        "final": bool(m.get("final")),
                     }
                 )
         # Audio + subtitle tracks (ordinals among their stream type — used to -map the selection).