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/ClientsideFriendsImport.cs

@@ -1,5 +1,8 @@
+using System.Text.Json;
 using System.Text.Json.Serialization;
+using Prospect.Server.Api.Services.Auth.Extensions;
 using Prospect.Server.Api.Services.CloudScript;
+using Prospect.Server.Api.Services.UserData;
 
 public class ClientsideFriendsImportRequest
 {
@@ -18,15 +21,33 @@ public class ClientsideFriendsImportResponse
 public class ClientsideFriendsImportFunction : ICloudScriptFunction<ClientsideFriendsImportRequest, ClientsideFriendsImportResponse>
 {
     private readonly ILogger<ClientsideFriendsImportFunction> _logger;
+    private readonly IHttpContextAccessor _httpContextAccessor;
+    private readonly UserDataService _userDataService;
 
-    public ClientsideFriendsImportFunction(ILogger<ClientsideFriendsImportFunction> logger)
+    public ClientsideFriendsImportFunction(ILogger<ClientsideFriendsImportFunction> logger, IHttpContextAccessor httpContextAccessor, UserDataService userDataService)
     {
         _logger = logger;
+        _httpContextAccessor = httpContextAccessor;
+        _userDataService = userDataService;
     }
 
     public async Task<ClientsideFriendsImportResponse> ExecuteAsync(ClientsideFriendsImportRequest request)
     {
-        _logger.LogInformation("Processing friends import");
+        var context = _httpContextAccessor.HttpContext;
+        if (context == null)
+        {
+            throw new CloudScriptException("CloudScript was not called within a http request");
+        }
+        var userId = context.User.FindAuthUserId();
+
+        // Persist the platform friend ids the client imports (Steam ids) so GetFriendList
+        // can later resolve which of them actually play on this server.
+        var ids = request.UserIDs ?? Array.Empty<string>();
+        await _userDataService.UpdateAsync(userId, userId, new Dictionary<string, string>
+        {
+            ["ImportedSteamFriends"] = JsonSerializer.Serialize(ids),
+        });
+        _logger.LogInformation("Imported {Count} {Platform} friend id(s) for user {User}", ids.Length, request.Platform, userId);
 
         return new ClientsideFriendsImportResponse{};
     }