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/Migrations/20260627171751_AddGuildInviteSurrogatePk.cs

@@ -0,0 +1,59 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+using Npgsql.EntityFrameworkCore.PostgreSQL.Metadata;
+
+#nullable disable
+
+namespace SVSim.Database.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddGuildInviteSurrogatePk : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropPrimaryKey(
+                name: "PK_GuildInvites",
+                table: "GuildInvites");
+
+            migrationBuilder.AddColumn<long>(
+                name: "Id",
+                table: "GuildInvites",
+                type: "bigint",
+                nullable: false,
+                defaultValue: 0L)
+                .Annotation("Npgsql:ValueGenerationStrategy", NpgsqlValueGenerationStrategy.IdentityByDefaultColumn);
+
+            migrationBuilder.AddPrimaryKey(
+                name: "PK_GuildInvites",
+                table: "GuildInvites",
+                column: "Id");
+
+            migrationBuilder.CreateIndex(
+                name: "IX_GuildInvites_GuildId_InviteeViewerId",
+                table: "GuildInvites",
+                columns: new[] { "GuildId", "InviteeViewerId" },
+                unique: true);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropPrimaryKey(
+                name: "PK_GuildInvites",
+                table: "GuildInvites");
+
+            migrationBuilder.DropIndex(
+                name: "IX_GuildInvites_GuildId_InviteeViewerId",
+                table: "GuildInvites");
+
+            migrationBuilder.DropColumn(
+                name: "Id",
+                table: "GuildInvites");
+
+            migrationBuilder.AddPrimaryKey(
+                name: "PK_GuildInvites",
+                table: "GuildInvites",
+                columns: new[] { "GuildId", "InviteeViewerId" });
+        }
+    }
+}