fix(guild): emit is_friend / is_friend_apply in /guild/info members

GuildMemberInfo.cs:48-49 re-reads json["is_friend"] / json["is_friend_apply"]
UNGUARDED on top of the base-class UserInfoBase's guarded read. Our DTO had
them as int? with global WhenWritingNull → keys omitted → LitJson threw
KeyNotFoundException, crashing the client right after guild create.

Flip the fields to non-nullable int with JsonIgnoreCondition.Never so they
always ship, and wire a batched IFriendService.GetFriendRelationsAsync
lookup so the values are actually populated (single Contains-batched query
per relation table).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

SVSim.Database/Services/Friend/IFriendService.cs

@@ -27,4 +27,12 @@ public interface IFriendService
 
     /// <summary>Deletes both directions of the friendship (A→B and B→A).</summary>
     Task RejectFriendAsync(long viewerId, int targetViewerId, CancellationToken ct);
+
+    /// <summary>
+    /// Batched caller-relative friend-relation lookup. For each id in <paramref name="otherViewerIds"/>
+    /// returns whether the caller is already friends with them and/or has an outgoing pending apply
+    /// to them. Self and unknown ids resolve to <c>(false, false)</c>.
+    /// </summary>
+    Task<IReadOnlyDictionary<long, FriendRelation>> GetFriendRelationsAsync(
+        long viewerId, IReadOnlyList<long> otherViewerIds, CancellationToken ct);
 }