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.
Players could never claim monthly guild items because the handler
always returned 0x01 (claimed). Now tracks per-character per-type
(standard/HLC/EXC) claim timestamps in the stamps table, comparing
against the current month boundary to determine claim eligibility.
Adds MonthStart() to gametime, extends StampRepo with
GetMonthlyClaimed/SetMonthlyClaimed, and includes schema migration
31-monthly-items.sql.
fmt.Sprintf inside zap logger calls defeats structured logging,
making log aggregation and filtering harder. All 6 sites now use
proper zap fields (zap.Uint32, zap.Uint8, zap.String).
LoopDelay had no viper.SetDefault, so omitting it from config.json
caused a zero-value (0 ms) busy-loop in the recv loop. Default is
now 50 ms, matching config.example.json.
