fix: fetch ERP months sequentially to avoid 500 errors

The ERP API can't handle concurrent requests — switch from batched
parallel (4 at a time) to sequential fetching.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/services/erpService.ts

@@ -85,18 +85,12 @@ export async function fetchFromERP(): Promise<MuseumRecord[]> {
   console.log('Fetching from ERP API via proxy...');
   const months = generateMonthBoundaries(2024, 1);
 
-  // Fetch all months in parallel (batched in groups of 4 to avoid overwhelming)
-  const batchSize = 4;
+  // Fetch months sequentially — the ERP API doesn't handle concurrent requests well
   const allSales: ERPSaleRecord[] = [];
 
-  for (let i = 0; i < months.length; i += batchSize) {
-    const batch = months.slice(i, i + batchSize);
-    const results = await Promise.all(
-      batch.map(([start, end]) => fetchChunk(start, end))
-    );
-    for (const chunk of results) {
-      allSales.push(...chunk);
-    }
+  for (const [start, end] of months) {
+    const chunk = await fetchChunk(start, end);
+    allSales.push(...chunk);
   }
 
   console.log(`Fetched ${allSales.length} transactions, aggregating...`);