feat(api): also clear cache when leaving a guild

Catalogger.Backend/Cache/InMemoryCache/InMemoryMemberCache.cs

@@ -145,4 +145,13 @@ public class InMemoryMemberCache(IDiscordRestGuildAPI guildApi, ILogger logger)
     ) => Task.FromResult<IEnumerable<(string, string)>>([]);
 
     public Task TryRemoveMemberNameAsync(Snowflake guildId, string username) => Task.CompletedTask;
+
+    public Task RemoveAllMembersAsync(Snowflake guildId)
+    {
+        foreach (var kv in _members.Where(kv => kv.Key.GuildId == guildId))
+            _members.Remove(kv.Key, out _);
+
+        _guilds.Remove(guildId, out _);
+        return Task.CompletedTask;
+    }
 }