fix(channelserver): handle ignored DB errors and cache userID on session

Silently ignored DB errors in handlers could cause data loss (frontier
point transactions completing without DB writes), reward duplication
(stamp exchange granting items on failed UPDATE), and crashes (tower
mission page=0 causing index-out-of-bounds). House access state
defaulting to 0 on DB failure also bypassed all access controls.

HIGH risk fixes:
- frontier point buy/sell now fails with ACK on DB error
- stamp exchange/stampcard abort on failed UPDATE
- guild meal INSERT returns fail ACK instead of orphaned ID 0
- mercenary/airou creation aborts on failed sequence nextval

MEDIUM risk fixes:
- tower mission page clamped to >= 1 preventing array underflow
- tower RP donation returns early on failed guild state read
- house state defaults to 2 (password-protected) on DB failure
- playtime read failure logged instead of silently resetting RP

Also cache userID on Session at login time, eliminating ~25 redundant
subqueries of the form WHERE u.id=(SELECT c.user_id FROM characters
c WHERE c.id=$1) across shop, gacha, command, and distitem handlers.

server/channelserver/handlers_shop.go

@@ -263,9 +263,17 @@ func handleMsgMhfExchangeFpoint2Item(s *Session, p mhfpacket.MHFPacket) {
 	pkt := p.(*mhfpacket.MsgMhfExchangeFpoint2Item)
 	var balance uint32
 	var itemValue, quantity int
-	_ = s.server.db.QueryRow("SELECT quantity, fpoints FROM fpoint_items WHERE id=$1", pkt.TradeID).Scan(&quantity, &itemValue)
+	if err := s.server.db.QueryRow("SELECT quantity, fpoints FROM fpoint_items WHERE id=$1", pkt.TradeID).Scan(&quantity, &itemValue); err != nil {
+		s.logger.Error("Failed to read fpoint item cost", zap.Error(err))
+		doAckSimpleFail(s, pkt.AckHandle, nil)
+		return
+	}
 	cost := (int(pkt.Quantity) * quantity) * itemValue
-	_ = s.server.db.QueryRow("UPDATE users u SET frontier_points=frontier_points::int - $1 WHERE u.id=(SELECT c.user_id FROM characters c WHERE c.id=$2) RETURNING frontier_points", cost, s.charID).Scan(&balance)
+	if err := s.server.db.QueryRow("UPDATE users SET frontier_points=frontier_points::int - $1 WHERE id=$2 RETURNING frontier_points", cost, s.userID).Scan(&balance); err != nil {
+		s.logger.Error("Failed to deduct frontier points", zap.Error(err))
+		doAckSimpleFail(s, pkt.AckHandle, nil)
+		return
+	}
 	bf := byteframe.NewByteFrame()
 	bf.WriteUint32(balance)
 	doAckSimpleSucceed(s, pkt.AckHandle, bf.Data())
@@ -275,9 +283,17 @@ func handleMsgMhfExchangeItem2Fpoint(s *Session, p mhfpacket.MHFPacket) {
 	pkt := p.(*mhfpacket.MsgMhfExchangeItem2Fpoint)
 	var balance uint32
 	var itemValue, quantity int
-	_ = s.server.db.QueryRow("SELECT quantity, fpoints FROM fpoint_items WHERE id=$1", pkt.TradeID).Scan(&quantity, &itemValue)
+	if err := s.server.db.QueryRow("SELECT quantity, fpoints FROM fpoint_items WHERE id=$1", pkt.TradeID).Scan(&quantity, &itemValue); err != nil {
+		s.logger.Error("Failed to read fpoint item value", zap.Error(err))
+		doAckSimpleFail(s, pkt.AckHandle, nil)
+		return
+	}
 	cost := (int(pkt.Quantity) / quantity) * itemValue
-	_ = s.server.db.QueryRow("UPDATE users u SET frontier_points=COALESCE(frontier_points::int + $1, $1) WHERE u.id=(SELECT c.user_id FROM characters c WHERE c.id=$2) RETURNING frontier_points", cost, s.charID).Scan(&balance)
+	if err := s.server.db.QueryRow("UPDATE users SET frontier_points=COALESCE(frontier_points::int + $1, $1) WHERE id=$2 RETURNING frontier_points", cost, s.userID).Scan(&balance); err != nil {
+		s.logger.Error("Failed to credit frontier points", zap.Error(err))
+		doAckSimpleFail(s, pkt.AckHandle, nil)
+		return
+	}
 	bf := byteframe.NewByteFrame()
 	bf.WriteUint32(balance)
 	doAckSimpleSucceed(s, pkt.AckHandle, bf.Data())