feat: add topic filter dropdown in course schedule and detail page

[ahmed-arb]

Add Topic Filter Dropdown to Course Schedule and Details Page

Summary

This PR adds a new "Topic" dropdown field to the course schedule and details page, allowing course authors to categorize courses by topic.

Changes

• API Response Enhancement: Added topic field to course details GET response, fetched from course_block.other_course_settings
• Settings Endpoint: Added topic_options to course settings serializer and endpoint to populate dropdown choices
• Update Handler: Integrated topic saving in CourseDetails.update_from_json() by calling update_other_course_settings() utility function

Technical Details

• Topic is stored in course_block.other_course_settings
• Topic options are retrieved via get_topics() utility from the wikimedia_general app
• All imports from custom app are lazy-loaded to prevent initialization issues

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Related PRs:
openedx-wikilearn-features: wikimedia/openedx-wikilearn-features#17
frontend-app-authoring: edly-io/frontend-app-authoring#5

Ticket: #569