Rebuilt app.py's register route to use db_op, and made changes to db_…

[alkatra]

1. Rebuilt app.py's register route to use db_op
2. Made changes to database_operation.py to check if user exists

Description

Rebuilt the register route in app.py to use the new db_op function for improved database operations. Updated database_operation.py to include a check for existing users, addressing issue #BE/FE: Rebuild register route.

Fixes #BE/FE: Rebuild register route.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• [] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Tested using sample API calls.

Testing Checklist

• Tested in latest Chrome
• Tested in latest Safari
• Tested in latest Firefox

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have requested a review from NAMES HERE on the Pull Request