feat(social): friends list resolution + fix ping request 500

- RequestUsersPingRequest: the client sends ping=2147483648 (int.MaxValue+1)
  for "no ping", which overflowed the int field and made every ping request
  return 500. Widened to long.
- ClientsideFriendsImport: persist the imported Steam friend ids per user.
- GetFriendList: resolve those ids to the players that exist on this server
  (batch lookup by Steam auth key) and return them as friends. Response
  shape is best-effort pending in-game confirmation.
- DbUserService.FindManyAsync: batch-resolve auth keys to players.

First step of the station lobby (see friends on the server). Squad
formation / invite flow to follow once friends are visible.

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

src/Prospect.Server.Api/Services/CloudScript/Functions/RequestUsersPingRequest.cs

@@ -6,7 +6,7 @@ public class RegionObject
     [JsonPropertyName("region")]
     public string Region { get; set; }
     [JsonPropertyName("ping")]
-    public int Ping { get; set; }
+    public long Ping { get; set; } // client sends 2147483648 (int.MaxValue+1) for "no ping" -> must be long or deserialization 500s
     [JsonPropertyName("instanceType")]
     public int InstanceType { get; set; }
 }