feat(guild): invite state machine

Add surrogate PK (IDENTITY bigint) to GuildInvite for wire invite_id.
Implement InviteAsync, CancelInviteAsync, RejectInviteAsync,
ListOutgoingInvitesAsync, ListInvitedGuildsAsync in GuildService.
Wire 5 endpoints: invite_user_list, invited_guild_list, invite,
cancel_invite, reject_invite in GuildController.
Migration: AddGuildInviteSurrogatePk — drops composite PK, adds Id
IDENTITY column, adds unique index on (GuildId, InviteeViewerId).
6 integration tests in GuildInviteFlowTests all green.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

SVSim.Database/Entities/Guild/GuildInvite.cs

@@ -2,6 +2,12 @@ namespace SVSim.Database.Entities.Guild;
 
 public class GuildInvite
 {
+    /// <summary>
+    /// Auto-increment surrogate PK. Used as the wire <c>invite_id</c> returned to the client
+    /// and echoed back in cancel_invite / reject_invite requests.
+    /// </summary>
+    public long Id { get; set; }
+
     public int GuildId { get; set; }
     public long InviteeViewerId { get; set; }
     public long InviterViewerId { get; set; }