fix: serialize concurrent buys per market + always verify on-chain payout before redeem

executeBuy (race condition):
- Multiple WebSocket events for the same market can arrive concurrently.
  All calls saw no existing position and all proceeded to buy → 3x fills.
- Added _buyQueue (Map<conditionId, Promise>) that chains each buy for
  the same market after the previous one. Second call now sees the filled
  position and respects maxPositionSize.

redeemer (gas estimation error):
- Gamma API can return resolved=true before payoutDenominator is written
  on-chain. Calling redeemPositions when payoutDenominator==0 causes the
  contract to revert → UNPREDICTABLE_GAS_LIMIT from ethers.js.
- Now always verify on-chain payout after the API check. If on-chain
  payout not set yet, skip and retry next interval.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/services/redeemer.js

@@ -140,17 +140,22 @@ export async function checkAndRedeemPositions() {
 
     for (const position of positions) {
         try {
-            // 1. Check via Gamma API
+            // 1. Quick check via Gamma API (low cost)
             const resolution = await checkMarketResolution(position.conditionId);
-            const isResolved = resolution?.resolved;
+            const apiResolved = resolution?.resolved;
 
-            if (!isResolved) {
-                // 2. Fallback: on-chain check
-                const onChain = await checkOnChainPayout(position.conditionId);
-                if (!onChain.resolved) continue;
-                logger.info(`Market resolved on-chain: ${position.market}`);
-            } else {
-                logger.info(`Market resolved: ${position.market}`);
+            if (!apiResolved) continue; // Not resolved yet — check again next interval
+
+            logger.info(`Market resolved via API: ${position.market}`);
+
+            // 2. ALWAYS verify on-chain payout before calling redeemPositions.
+            //    Gamma API can report "resolved" before payoutDenominator is written
+            //    on-chain. Calling redeemPositions with payoutDenominator == 0 causes
+            //    the contract to revert → gas estimation failure.
+            const onChain = await checkOnChainPayout(position.conditionId);
+            if (!onChain.resolved) {
+                logger.info(`On-chain payout not set yet for ${position.market} — will retry next interval`);
+                continue;
             }
 
             // 3. Simulate or execute real redeem