fix: remove duplicate Mongoose index on username and email fields

[Ambuj123554]

Issue:

Fixes #3752

When running the backend locally, Mongoose throws duplicate index
warnings for both the username and email fields on startup.
This was caused by having both unique: true in the schema field
definition AND a separate userSchema.index() call for the same
fields, resulting in two index definitions for each field.

Demo:

No UI changes — this is a backend fix. The Mongoose warnings no
longer appear on server startup after this fix.

Changes:

• Removed unique: true from the username field definition
• Removed unique: true from the email field definition
• Uniqueness and case-insensitive collation is now handled solely
  by the existing userSchema.index() calls at the bottom of the
  file, which already include the correct collation settings

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test) — note: npm run test
  fails on Windows due to NODE_ENV syntax, unrelated to this fix
• has no typecheck errors (npm run typecheck)
• is from a uniquely-named feature branch and is up to date
  with the develop branch.
• is descriptively named and links to an issue number — Fixes Duplicate Mongoose Index Warning for username field on Server Startup #3752
• meets the standards outlined in the accessibility guidelines