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.
