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/Database/DbUserService.cs

@@ -40,7 +40,21 @@ public class DbUserService : BaseDbService<PlayFabUser>
 
     public async Task<PlayFabUser> FindOrCreateAsync(PlayFabUserAuthType type, string key)
     {
-        return await FindAsync(type, key) ?? 
+        return await FindAsync(type, key) ??
                await CreateAsync(type, key);
     }
+
+    // Resolve several auth keys (e.g. a batch of Steam IDs) to the players that exist on
+    // this server in a single query. Used by the friends list to keep only the imported
+    // Steam friends who have actually played here.
+    public async Task<List<PlayFabUser>> FindManyAsync(PlayFabUserAuthType type, IReadOnlyCollection<string> keys)
+    {
+        if (keys == null || keys.Count == 0)
+        {
+            return new List<PlayFabUser>();
+        }
+        var wanted = new HashSet<string>(keys);
+        return await Collection.Find(user => user.Auth.Any(auth =>
+            auth.Type == type && wanted.Contains(auth.Key))).ToListAsync();
+    }
 }