Fix daily leaderboard delta calculation for single stats

- Use most recent stat before 24h window instead of oldest ever
- Consistent logic for both ELO and XP calculations
- Shows actual gain since last activity

🤖 Generated with [Claude Code](https://claude.ai/code)

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

Author: codergautam

api/leaderboard.js

@@ -65,26 +65,31 @@ async function getDailyLeaderboard(isXp = true) {
         statCount: { $sum: 1 }
       }
     },
-    // STEP 3.5: For users with only one stat in 24h, get their oldest ever stat
+    // STEP 3.5: For users with only one stat in 24h, get their most recent stat before 24h window
     {
       $lookup: {
         from: 'userstats',
-        let: { userId: '$_id', statCount: '$statCount' },
+        let: { 
+          userId: '$_id', 
+          statCount: '$statCount',
+          dayAgo: new Date(Date.now() - (24 * 60 * 60 * 1000))
+        },
         pipeline: [
           {
             $match: {
               $expr: {
                 $and: [
                   { $eq: ['$userId', '$$userId'] },
-                  { $eq: ['$$statCount', 1] }
+                  { $eq: ['$$statCount', 1] },
+                  { $lt: ['$timestamp', '$$dayAgo'] }
                 ]
               }
             }
           },
-          { $sort: { timestamp: 1 } },
+          { $sort: { timestamp: -1 } },
           { $limit: 1 }
         ],
-        as: 'oldestStat'
+        as: 'previousStat'
       }
     },
     // STEP 4: Calculate deltas and add user lookup in single stage
@@ -116,8 +121,8 @@ async function getDailyLeaderboard(isXp = true) {
             then: { $subtract: [`$latestStat.${scoreField}`, `$earliestStat.${scoreField}`] },
             else: {
               $cond: {
-                if: { $gt: [{ $size: '$oldestStat' }, 0] },
-                then: { $subtract: [`$latestStat.${scoreField}`, { $arrayElemAt: [`$oldestStat.${scoreField}`, 0] }] },
+                if: { $gt: [{ $size: '$previousStat' }, 0] },
+                then: { $subtract: [`$latestStat.${scoreField}`, { $arrayElemAt: [`$previousStat.${scoreField}`, 0] }] },
                 else: 0
               }
             }