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.

CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- Return/Rookie Guild system: new players are automatically placed in a temporary rookie guild (`return_type=1`) and returning players in a comeback guild (`return_type=2`) via `MSG_MHF_ENTRY_ROOKIE_GUILD`. Players graduate (leave) via `OperateGuildGraduateRookie`/`OperateGuildGraduateReturn`. Guild info response now reports `isReturnGuild` correctly. Database migration `0014_return_guilds` adds `return_type` to the `guilds` table.
 - `saveutil` admin CLI (`cmd/saveutil/`): `import`, `export`, `grant-import`, and `revoke-import` commands for transferring character save data between server instances without touching the database manually.
 - `POST /v2/characters/{id}/import` API endpoint: player-facing save import gated behind a one-time admin-granted token (generated by `saveutil grant-import`). Token expires after a configurable TTL (default 24 h).
 - Database migration `0013_save_transfer`: adds `savedata_import_token` and `savedata_import_token_expiry` columns to the `characters` table.