feat: add PartnerCatalogCourseSerializer

[Alec4r]

This PR introduces several improvements to the serializers used in the Corporate Partner Catalog workflow. These changes add new fields, improve nested data responses, and ensure consistent ordering for catalog courses.

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✅ Changes Included

1. PartnerCatalogCourseSerializer Improvements

• Added order (IntegerField, read-only) mapped from position.
• Added course_key (SerializerMethodField) exposing course_overview.id.
• Added ordering = ["position"] inside Meta to ensure consistent ordering.

2. PartnerCatalogSerializer Updates

• Replaced PrimaryKeyRelatedField for partner with the full PartnerSerializer to return nested partner details.

• Added steps using PartnerCatalogCourseSerializer (many=True, read-only).

• Introduced new fields:

  • image = ImageField(read_only=True)
  • email_regexes = ListField(child=CharField(max_length=500))

• Updated the fields and read_only_fields lists accordingly.

3. General Cleanup

• Minor reordering for clarity and serializer structure consistency.

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🎯 Motivation

• Provide richer and more complete catalog data for the frontend.
• Support UI features that require partner details, catalog steps, and additional metadata.
• Improve payload consistency and ordering of catalog courses.

[Ang-m4]

LGTM