pindexer: dex-explorer handle sparse block snapshots for summary

This looks backwards for the current price and liquidity,
allowing us to handle cases where snapshots are sparse, like on the
testnet.

crates/bin/pindexer/src/dex_ex/mod.rs

@@ -435,8 +435,8 @@ mod summary {
         now AS (
             SELECT price, liquidity            
             FROM snapshots
-            WHERE time >= $3
-            ORDER BY time ASC
+            WHERE time <= $3
+            ORDER BY time DESC
             LIMIT 1
         ),
         sums AS (