feat(guild): implement rookie and return guild assignment

New/returning players are now auto-assigned to temporary holding guilds
on MSG_MHF_ENTRY_ROOKIE_GUILD (pkt.Unk=0 → rookie guild, ≥1 → comeback
guild). Guilds are created on demand and capped at 60 members. Players
leave via the OperateGuildGraduateRookie/Return actions. The guild info
response now reports isReturnGuild from the DB instead of hardcoded false.
Migration 0014_return_guilds adds return_type to the guilds table.

server/migrations/sql/0014_return_guilds.sql

@@ -0,0 +1,5 @@
+BEGIN;
+
+ALTER TABLE public.guilds ADD COLUMN IF NOT EXISTS return_type SMALLINT NOT NULL DEFAULT 0;
+
+COMMIT;