feat(guild): add Guild aggregate schema + migration

SVSim.Database/Entities/Guild/Guild.cs

@@ -0,0 +1,17 @@
+using SVSim.Database.Models;
+
+namespace SVSim.Database.Entities.Guild;
+
+public class Guild
+{
+    public int GuildId { get; set; }                    // server-generated 9-digit
+    public string Name { get; set; } = "";
+    public string Description { get; set; } = "";
+    public long LeaderViewerId { get; set; }
+    public long EmblemId { get; set; }
+    public GuildActivity Activity { get; set; }
+    public GuildJoinCondition JoinCondition { get; set; }
+    public DateTime CreatedAt { get; set; }
+    public DateTime? BreakupAt { get; set; }            // soft-delete on breakup
+    public List<GuildMember> Members { get; set; } = new();
+}