Implement updateDailySummary() aggregation function

Add daily summary aggregation that tracks apartment inventory changes:
- Upsert summaries by date key for idempotent operation
- Track new_units, rented_units, and stale_units arrays with counts
- Calculate net_change (new - rented) and total_available_today
- Fetch previous day summary for total_available_yesterday
- Compute turnover_rate as percentage of rented vs yesterday available
- Handle edge cases: zero division, missing previous day, month boundaries
- Full test coverage with 22 tests across all functional areas

services/scraperService.js

@@ -369,6 +369,21 @@ function convertDataTypes(unit) {
   };
 }
 
+// ============================================================
+// Date Helpers
+// ============================================================
+
+/**
+ * Calculate yesterday's date from a given date string.
+ * @param {string} dateStr - Date in YYYY-MM-DD format
+ * @returns {string} Yesterday's date in YYYY-MM-DD format
+ */
+function getYesterday(dateStr) {
+  const date = new Date(dateStr + 'T00:00:00Z');
+  date.setUTCDate(date.getUTCDate() - 1);
+  return date.toISOString().split('T')[0];
+}
+
 // ============================================================
 // Database Operations
 // ============================================================
@@ -535,6 +550,75 @@ async function markStaleUnits(db, currentUnitCodes, date, logger) {
   }
 }
 
+/**
+ * Update daily summary document.
+ * Upserts by date field (YYYY-MM-DD format).
+ * Fetches yesterday's summary for comparison metrics.
+ * Calculates turnover_rate as (rentedUnits / yesterdayTotal * 100).
+ *
+ * @param {Db} db - MongoDB database instance
+ * @param {Object} summaryData - Summary data
+ * @param {string} summaryData.date - Date in YYYY-MM-DD format
+ * @param {Array<string>} summaryData.newUnits - Unit codes added today
+ * @param {Array<string>} summaryData.rentedUnits - Unit codes removed today
+ * @param {number} summaryData.staleUnitsCount - Count of stale units
+ * @param {number} summaryData.totalAvailable - Total available units today
+ * @param {Object} logger - Logger instance
+ * @returns {Promise<Object>} MongoDB updateOne result
+ */
+async function updateDailySummary(db, summaryData, logger) {
+  const collection = db.collection(config.COLLECTIONS.DAILY_SUMMARIES);
+  const { date, newUnits, rentedUnits, staleUnitsCount, totalAvailable } = summaryData;
+
+  // Calculate yesterday's date for comparison
+  const yesterday = getYesterday(date);
+
+  // Get yesterday's summary for comparison
+  const yesterdaySummary = await collection.findOne({ date: yesterday });
+  const yesterdayTotal = yesterdaySummary?.total_available_today || 0;
+
+  const summary = {
+    date,
+    timestamp: new Date().toISOString(),
+    new_units: newUnits,
+    rented_units: rentedUnits,
+    stale_units: [], // Stale units list is not tracked per PRD
+    new_units_count: newUnits.length,
+    rented_units_count: rentedUnits.length,
+    stale_units_count: staleUnitsCount,
+    net_change: newUnits.length - rentedUnits.length,
+    total_available_today: totalAvailable,
+    total_available_yesterday: yesterdayTotal,
+    turnover_rate: yesterdayTotal > 0
+      ? Math.round((rentedUnits.length / yesterdayTotal) * 10000) / 100
+      : 0
+  };
+
+  try {
+    const result = await collection.updateOne(
+      { date },
+      { $set: summary },
+      { upsert: true }
+    );
+
+    logger.info('Daily summary updated', {
+      date,
+      newUnits: newUnits.length,
+      rentedUnits: rentedUnits.length,
+      totalAvailable
+    });
+
+    return result;
+
+  } catch (error) {
+    logger.error('Failed to update daily summary', {
+      errorType: error.name,
+      errorMessage: error.message
+    });
+    throw error;
+  }
+}
+
 module.exports = {
   fetchPage,
   parseUnits,
@@ -542,7 +626,9 @@ module.exports = {
   upsertUnits,
   insertPrices,
   markStaleUnits,
+  updateDailySummary,
   // Export helpers for testing
+  getYesterday,
   parseInteger,
   parsePositiveInteger,
   parseIntegerOrString,