Performance Issue: No Index on Email/Username Lookup Fields

Description

User lookup queries (login, profile fetch, session validation) perform full collection/table scans because the email and username fields lack database indexes. With a growing user base, this causes response time degradation on every authenticated request.

Steps to Reproduce

1. Insert 10,000+ user records into the database
2. Enable query explain/analyze (MongoDB: db.users.find({email: 'x'}).explain("executionStats"))
3. Observe: COLLSCAN (full collection scan) instead of IXSCAN (index scan)
4. Execution time scales linearly with user count

Root Cause

The user model/schema does not define indexes on frequently queried fields.

Impact

• Login response time of 100ms+ per query at scale
• Every page requiring authentication triggers at least one user lookup
• Database becomes the bottleneck under moderate traffic

Proposed Fix

Add unique indexes to the user model:

// Mongoose schema
const userSchema = new Schema({
  email: { type: String, required: true, unique: true, index: true },
  username: { type: String, required: true, unique: true, index: true },
  // ...
});

The unique: true constraint also implicitly creates an index. For compound queries, add a compound index.

I would like to implement this and add a migration script if assigned.