Refactor: Rename route from `/plans` to `/billing-plans` for clarity according to the swagger docs

[idehen-divine]

Description

Renaming the route resource from /plans to /billing-plans to improve naming consistency and clarity. This ensures better maintainability and aligns with our naming conventions.

Related Issue (Link to Github issue)

N/A (New refactor request)

Motivation and Context

The current route naming is inconsistent/confusing. Renaming it to /billing-plans makes it more intuitive and aligns with the rest of our API structure.

How Has This Been Tested?

• Updated all references in controllers and views
• Checked for any broken links or dependencies
• Verified functionality in Postman/browser
• Ran existing test suite to confirm no regressions

Screenshots (if appropriate - Postman, etc):

N/A

Types of changes

• Refactor (non-breaking change that improves maintainability)
• 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 change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[idehen-divine]