[REFACTOR] Move Hardcoded Email Subjects to Constants Across the User Related Files

[ThisIsMani]

Feature Description

In the current implementation, email subjects are hardcoded at multiple places across the codebase when constructing email content. To improve maintainability and avoid magic strings, each unique subject should be moved to its own constant. The goal of this issue is to identify all the places where email subjects are hardcoded, create a corresponding constant for each one, and replace the hardcoded values with the newly defined constants.

Possible Implementation

• Search the codebase for all instances where email subjects are hardcoded. Example: https://github.com/juspay/hyperswitch/blob/b2364414edab6d44e276f2133bc988610693f155/crates/router/src/core/user.rs#L707
• For each unique subject, create a new constant in crates/router/src/consts/user.rs.
• Refactor the code by replacing the hardcoded subjects with their respective constants.

Have you spent some time checking if this feature request has been raised before?

• [X] I checked and didn't find a similar issue

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