Add scraper indexes and switch to validation collection names

Add createScraperIndexes() to create indexes on the scraper_runs
collection: a compound index on status+startedAt for active job queries
and a descending index on startedAt for recent run lookups.

Change default collection names from production tables
(units_migration_test, unit_prices_migration_test) to dedicated
validation collections (units_scraper, unit_prices_scraper). This
enables the Node.js scraper to run in parallel with the existing Python
scraper during validation without interfering with production data.

Update all scraper test files to reference the new default collection
names.

services/scraperService.js

@@ -730,6 +730,36 @@ async function recordScraperRun(db, runData, logger) {
   }
 }
 
+// ============================================================
+// Index Management
+// ============================================================
+
+/**
+ * Create indexes on the scraper_runs collection.
+ * Should be called once during application startup.
+ * Idempotent - safe to call multiple times.
+ *
+ * @param {Db} db - MongoDB database instance
+ * @returns {Promise<void>}
+ */
+async function createScraperIndexes(db) {
+  const collection = db.collection(config.COLLECTIONS.SCRAPER_RUNS);
+
+  // Compound index for querying runs by status sorted by most recent
+  await collection.createIndex(
+    { status: 1, startedAt: -1 },
+    { name: 'status_startedAt' }
+  );
+
+  // Index for sorting all runs by start time (most recent first)
+  await collection.createIndex(
+    { startedAt: -1 },
+    { name: 'startedAt_desc' }
+  );
+
+  console.log('Scraper indexes created successfully');
+}
+
 // ============================================================
 // Main Orchestration Function
 // ============================================================
@@ -871,6 +901,7 @@ module.exports = {
   markStaleUnits,
   updateDailySummary,
   recordScraperRun,
+  createScraperIndexes,
   // Export helpers for testing
   getTodayUTC,
   getYesterdayUnitCodes,