fix: add lastSeen and visitCount to UserSchema to prevent silent data loss (JhaSourav07#3125)

## Description

This PR resolves the Silent Data Loss issue in the `/api/track-user`
endpoint caused by Mongoose's `strict: true` behavior.
Changes made:
- Added `lastSeen` (Date) and `visitCount` (Number) to the `UserSchema`
in `models/User.ts`.
- Updated the `IUser` TypeScript interface to include `lastSeen?: Date`
and `visitCount?: number` to maintain type safety.
This ensures that analytics metrics are no longer silently dropped
during upsert operations and are successfully persisted to MongoDB.

Fixes JhaSourav07#3015 

## Pillar

- [ ] 🎨 Pillar 1 — New Theme Design
- [ ] 📐 Pillar 2 — Geometric SVG Improvement
- [ ] 🕐 Pillar 3 — Timezone Logic Optimization
- [x] 🛠️ Other (Bug fix, refactoring, docs)

## Visual Preview

## Checklist before requesting a review:

- [x] I have read the `CONTRIBUTING.md` file.
- [x] I have tested these changes locally
(`localhost:3000/api/streak?user=YOUR_USERNAME`).
- [x] I have run `npm run format` and `npm run lint` locally and
resolved all errors (CI will fail otherwise).
- [x] My commits follow the Conventional Commits format (e.g.,
`feat(themes): ...`, `fix(calculate): ...`).
- [x] I have updated `README.md` if I added a new theme or URL
parameter.
- [x] I have started the repo.
- [x] I have made sure that i have only one commit to merge in this PR.
- [x] The SVG output matches the CommitPulse "premium quality" aesthetic
standard (no raw elements, smooth animations, correct fonts).
- [x] (Recommended) I joined the CommitPulse Discord community for
contributor discussions, mentorship, and faster PR support.

Author: JhaSourav07

models/User.ts

@@ -3,9 +3,11 @@ import mongoose, { Document, Model, Schema } from 'mongoose';
 export interface IUser extends Document {
   username: string;
   createdAt: Date;
+  lastSeen?: Date;
+  visitCount: number;
 }
 
-const UserSchema: Schema = new Schema({
+const UserSchema: Schema<IUser> = new Schema<IUser>({
   username: {
     type: String,
     required: true,
@@ -17,6 +19,13 @@ const UserSchema: Schema = new Schema({
     type: Date,
     default: Date.now,
   },
+  lastSeen: {
+    type: Date,
+  },
+  visitCount: {
+    type: Number,
+    default: 0,
+  },
 });
 
 export const User: Model<IUser> = mongoose.models.User || mongoose.model<IUser>('User', UserSchema);