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/Services/Database/DbUserService.cs

@@ -13,11 +13,17 @@ public class DbUserService : BaseDbService<PlayFabUser>
 
     public async Task<PlayFabUser> FindAsync(PlayFabUserAuthType type, string key)
     {
-        return await Collection.Find(user => user.Auth.Any(auth => 
-            auth.Type == type && 
+        return await Collection.Find(user => user.Auth.Any(auth =>
+            auth.Type == type &&
             auth.Key == key)).FirstOrDefaultAsync();
     }
 
+    // Resolve a player by its PlayFab id (used to display squad member names).
+    public async Task<PlayFabUser> FindByIdAsync(string id)
+    {
+        return await Collection.Find(user => user.Id == id).FirstOrDefaultAsync();
+    }
+
     private async Task<PlayFabUser> CreateAsync(PlayFabUserAuthType type, string key)
     {
         var user = new PlayFabUser