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>
