Replace db.Begin() with db.BeginTxx(context.Background(), nil) across all
8 remaining call sites in repo_guild.go, repo_guild_rp.go, repo_festa.go,
and repo_event.go. Use deferred Rollback() instead of explicit rollback
at each error return, eliminating 15 manual rollback calls.
The 1004-line monolith covered 11 game subsystems across 62 methods.
Split into 7 files by domain (RP, posts, alliances, adventures, hunts,
cooking) while keeping core CRUD/membership/scouts in the original.
Same package, receiver, and interface — no behavior changes.
