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>
1b7fa2d525