AAI-431: Enable user password change

[amandazhuyilan]

Description

AAI-432: Enable user password change

Changes

• Added a /me/password endpoint for users to change their passwords
• Updates the Auth0 user with the management API, and returns friendly errors
• Includes new PasswordChangeRequest schema, helper utilities, configuration fallbacks, and guidance in .env.example`
• Updated tests

Checklist

• I have commented my code, particularly in hard-to-understand areas
• I have added unit / integration tests that prove my fix is effective or that my feature works
• I have run all tests locally and they pass
• I have updated the documentation (if applicable)
• For any new secrets, I have updated the shared spreadsheet and the GitHub Secrets.

How to Test Manually (if necessary)

<Describe how reviewers can manually, if necessary, test the changes>

[amandazhuyilan]

This PR will be merged with #124