fix(nodes): use GREATEST for last_online merge on PostgreSQL

setRemoteTrafficLocked merged last_online with MAX(last_online, ?), which
is SQLite's two-argument scalar max. PostgreSQL's MAX() is aggregate-only,
so node traffic sync failed every cycle with "function max(bigint, unknown)
does not exist (SQLSTATE 42883)", flooding the logs.

Add a dialect-aware database.GreatestExpr helper (GREATEST on Postgres,
MAX on SQLite) and use it for the last_online merge. last_online is a
non-null int64, so the two functions are semantically identical here.

Closes #4633

web/service/inbound.go

@@ -1503,10 +1503,13 @@ func (s *InboundService) setRemoteTrafficLocked(nodeID int, snap *runtime.Traffi
 			}
 
 			if err := tx.Exec(
-				`UPDATE client_traffics
-				 SET up = ?, down = ?, enable = ?, total = ?, expiry_time = ?, reset = ?,
-				     last_online = MAX(last_online, ?)
-				 WHERE email = ?`,
+				fmt.Sprintf(
+					`UPDATE client_traffics
+					 SET up = ?, down = ?, enable = ?, total = ?, expiry_time = ?, reset = ?,
+					     last_online = %s
+					 WHERE email = ?`,
+					database.GreatestExpr("last_online", "?"),
+				),
 				cs.Up, cs.Down, cs.Enable, cs.Total, cs.ExpiryTime, cs.Reset,
 				cs.LastOnline, cs.Email,
 			).Error; err != nil {