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/FriendDtos.cs

@@ -82,3 +82,9 @@ public sealed record BattleParticipationContext(
     int BattleType,
     int DeckFormat,
     int TwoPickType);
+
+/// <summary>
+/// Caller-relative friend flags for another viewer. Used to badge rows in guild-member,
+/// invite-candidate, and join-request lists on the wire (<c>is_friend</c>, <c>is_friend_apply</c>).
+/// </summary>
+public sealed record FriendRelation(bool IsFriend, bool HasOutgoingApply);