SCRAPE-8: Implement markStaleUnits() to flag unavailable units

Add markStaleUnits() that marks units as unavailable when they are not
present in the current scrape results. Uses MongoDB updateMany with a
$nin filter on unit_code to efficiently update only units that were
previously available but no longer appear in the scraped data. Sets
available to false and records marked_stale_date.

Includes 18 tests covering filter criteria, $set fields, already
unavailable units, empty currentUnitCodes, error handling, logging,
and return value verification.

services/scraperService.js

@@ -493,12 +493,55 @@ async function insertPrices(db, units, date, logger) {
   }
 }
 
+/**
+ * Mark units not in current scrape as stale/unavailable.
+ * Uses updateMany to set available: false and marked_stale_date
+ * for all units whose unit_code is NOT in the current scrape
+ * and that are currently available.
+ *
+ * @param {Db} db - MongoDB database instance
+ * @param {Set<string>} currentUnitCodes - Unit codes from current scrape
+ * @param {string} date - Current date in YYYY-MM-DD format
+ * @param {Object} logger - Logger instance
+ * @returns {Promise<Object>} Update result
+ */
+async function markStaleUnits(db, currentUnitCodes, date, logger) {
+  const collection = db.collection(config.COLLECTIONS.UNITS);
+
+  try {
+    const result = await collection.updateMany(
+      {
+        unit_code: { $nin: [...currentUnitCodes] },
+        available: true
+      },
+      {
+        $set: {
+          available: false,
+          marked_stale_date: date
+        }
+      }
+    );
+
+    logger.info('Stale units marked', { count: result.modifiedCount });
+
+    return result;
+
+  } catch (error) {
+    logger.error('Failed to mark stale units', {
+      errorType: error.name,
+      errorMessage: error.message
+    });
+    throw error;
+  }
+}
+
 module.exports = {
   fetchPage,
   parseUnits,
   convertDataTypes,
   upsertUnits,
   insertPrices,
+  markStaleUnits,
   // Export helpers for testing
   parseInteger,
   parsePositiveInteger,