refactor(channelserver): extract StampRepository, DistributionRepository, and SessionRepository

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.

server/channelserver/session_lifecycle_integration_test.go

@@ -607,6 +607,9 @@ func createTestServerWithDB(t *testing.T, db *sqlx.DB) *Server {
 	server.towerRepo = NewTowerRepository(db)
 	server.rengokuRepo = NewRengokuRepository(db)
 	server.mailRepo = NewMailRepository(db)
+	server.stampRepo = NewStampRepository(db)
+	server.distRepo = NewDistributionRepository(db)
+	server.sessionRepo = NewSessionRepository(db)
 
 	return server
 }