fix(analytics): remove SessionSchema.index sessionId duplicate

- Line 49 has sessionId with unique: true (creates index automatically)
- Line 75 had redundant SessionSchema.index({ sessionId: 1 })
- Removed explicit index to eliminate Mongoose duplicate warning

src/models/Analytics.model.js

@@ -72,7 +72,7 @@ const SessionSchema = new mongoose.Schema({
   timestamps: true
 });
 
-SessionSchema.index({ sessionId: 1 });
+// sessionId already indexed via unique: true on line 49
 SessionSchema.index({ visitorId: 1, startTime: -1 });
 SessionSchema.index({ startTime: -1 });