fix(analytics): remove duplicate sessionId index definition

- SessionSchema had both 'unique: true' and 'index: true'
- unique already creates an index, making index redundant
- Resolves Mongoose warning about duplicate schema index

src/models/Analytics.model.js

@@ -46,7 +46,7 @@ PageViewSchema.index({ sessionId: 1, timestamp: -1 });
 
 // Session Schema
 const SessionSchema = new mongoose.Schema({
-  sessionId: { type: String, required: true, unique: true, index: true },
+  sessionId: { type: String, required: true, unique: true }, // unique already creates index
   visitorId: { type: String, required: true, index: true },
 
   startTime: { type: Date, default: Date.now },