feat(squad): in-memory squad/lobby state + wire squad functions

Adds SquadService (singleton, process-local) tracking squads by the
squadId the client shares across party members, plus a user->squad index
so TryGetCompleteSquadInfo (no squadId in request) can resolve the
caller's squad. Members are lazily registered on any squad call.

- TryGetCompleteSquadInfo: returns the caller's real squad.
- SquadMemberReadyForMatch: joins the squad, records readiness/map,
  returns the squad + whether everyone is ready.
- SquadMemberSelectedMap: records the member's selected map.
- DbUserService.FindByIdAsync: resolve display name for squad members.

Groundwork for the station lobby; assumes the client drives squads via
these calls with a shared squadId (to confirm with a 2-client session).
Real-time SignalR push (unknown event names) deferred; the squad is
carried in responses + TryGetCompleteSquadInfo for now.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/Prospect.Server.Api/Startup.cs

@@ -35,6 +35,8 @@ public class Startup
 
         services.AddSingleton<DbUserDataService>();
 
+        services.AddSingleton<Prospect.Server.Api.Services.Squad.SquadService>();
+
         services.AddHostedService<QosService>();
         services.AddSingleton<QosServer>();