feat: per-sub-room max players, editable in game and from admin

Player caps were two settings pretending to be one. RoomEntity.MaxCapacity is
what the ROOM advertises (and what matchmaking hands to
RoomInstance.MaxCapacity); RoomSceneEntity.MaxPlayers is what ONE sub-room caps
itself at. The details payload was reading its room-level MaxPlayers off
sub-room 0, which collapsed the two — editing a sub-room would have moved the
room's advertised capacity with it. It now reports MaxCapacity, and each entry
in SubRooms[] reports its own figure.

The in-game path already existed (the "Max Player Count in This Subroom"
slider). Admin had no way to reach it at all — only zip import ever set it — so
a cap could not be corrected on a room the admin does not own:

  GET api/admin/v1/rooms/{id}/subrooms
  PUT api/admin/v1/rooms/{id}/subrooms/{subRoomId}/maxplayers  {"MaxPlayers":n}

subRoomId is the sub-room's ORDER INDEX — the id the client and the rest of the
wire use — not the row's primary key. Row ids are serialised as strings because
the admin SPA is JavaScript and mangles integers past 2^53. Both paths clamp to
1..80, matching rooms/{id}/props: 0 would make a sub-room unjoinable, and a
shared range stops the two settings disagreeing about what is legal.

The test drives the client's exact wire rather than a convenient one:
NLDBPDCNNCF.MNBPCGJNLNP builds a BLNOGFGHIIF whose single JSON key is
"maxPlayers" and PUTs it — this route does NOT form-encode like most of its
siblings, so a form-bodied test would have passed while the real client's body
went unread. It also asserts the reply carries the new value, since the client
deserialises it back as FGCPNAACHIK.

DorkNet.Server/Controllers/API/Rooms/V2/RoomsController.cs

@@ -2143,9 +2143,12 @@ public class RoomsController(
             // name-keyed reader, presence does.
             DataBlob = dataBlobName,
             DataBlobHash = (string?)null,
-            // Room-level cap mirrors sub-room 0, which is where the client
-            // reads it from for a single-sub-room room.
-            MaxPlayers = sceneRows is { Count: > 0 } ? sceneRows[0].MaxPlayers : 8,
+            // The room-level cap is the room's own advertised capacity, which is
+            // what RoomEntity.MaxCapacity exists for and what matchmaking hands
+            // to RoomInstance.MaxCapacity. Each sub-room carries its own
+            // MaxPlayers in the SubRooms array — they are separate settings, so
+            // this must not be read off sub-room 0.
+            MaxPlayers = room.MaxCapacity,
             // ToxMod is Rec Room's hosted voice moderation. There is no such
             // service here, so it is off — the client only uses this to decide
             // whether to show the indicator.