Add daily price change to /available-units endpoint

Add yesterdayPrice and priceChangeToday fields to show price delta
between today and yesterday instead of high-low spread.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

server.js

@@ -291,7 +291,8 @@ app.get('/price-history', requireAuth, async (req, res) => {
 app.get('/available-units', requireAuth, async (req, res) => {
   try {
     const today = await getLatestDateWithData();
-    
+    const yesterday = getYesterdayDate(today);
+
     // Get units that have prices today (excluding furnished units)
     const availableUnits = await db.collection(UNITS_COLLECTION).aggregate([
       {
@@ -315,11 +316,17 @@ app.get('/available-units', requireAuth, async (req, res) => {
       },
       {
         $addFields: {
-          currentPrice: {
+          currentPriceDoc: {
             $arrayElemAt: [
               { $filter: { input: "$price_info", cond: { $eq: ["$$this.date_checked", today] } } },
               0
             ]
+          },
+          yesterdayPriceDoc: {
+            $arrayElemAt: [
+              { $filter: { input: "$price_info", cond: { $eq: ["$$this.date_checked", yesterday] } } },
+              0
+            ]
           }
         }
       },
@@ -408,6 +415,17 @@ app.get('/available-units', requireAuth, async (req, res) => {
           }
         }
       },
+      {
+        $addFields: {
+          priceChangeToday: {
+            $cond: {
+              if: { $and: [{ $ne: ["$currentPriceDoc.price", null] }, { $ne: ["$yesterdayPriceDoc.price", null] }] },
+              then: { $subtract: ["$currentPriceDoc.price", "$yesterdayPriceDoc.price"] },
+              else: null
+            }
+          }
+        }
+      },
       {
         $project: {
           unitCode: "$unit_code",
@@ -418,7 +436,9 @@ app.get('/available-units', requireAuth, async (req, res) => {
           community: "$community",
           available: "$available",
           soonest: "$soonest",
-          currentPrice: "$currentPrice.price",
+          currentPrice: "$currentPriceDoc.price",
+          yesterdayPrice: "$yesterdayPriceDoc.price",
+          priceChangeToday: { $ifNull: ["$priceChangeToday", null] },
           minPrice: "$priceStats.minPrice",
           maxPrice: "$priceStats.maxPrice",
           daysAvailable: {