fix: remove partners URLs annidation

[Ang-m4]

Description

This pull request updates the partners–catalogs API by removing the unnecessary /partners/... nesting.
Partners now live under their own URL root, and catalogs stay under /catalogs/..., where they can still be queried by partner.

Key changes

• Removed the /partners/... nesting from the catalog endpoints.
• Added a new filter to query catalogs by partner_id.
• Kept the partner view as a base endpoint for any future partner-related implementations.
• Adjusted the partners view to only return partners where the current manager is a member.

Important

All the endpoints regarding learners, courses and so on, are still working the same

How to test

1. Run your Tutor environment.
2. Make sure you have some partners and catalogs in the system.
3. Hit the /catalogs/ API and filter by partner using the ?partner=<id> parameter, for example:

.../catalogs/?partner=1