Eliminate 18 direct s.server.db calls from handlers_items.go,
handlers_distitem.go, and handlers_session.go by moving queries into
dedicated repository types.
New repositories:
- StampRepository (7 methods, stamps table)
- DistributionRepository (4 methods, distribution/distribution_items)
- SessionRepository (4 methods, sign_sessions/servers)
Also adds ClearTreasureHunt and InsertKillLog to GuildRepository,
which already owns those tables for read operations.
