[Follow-up #255] Add private user tests for PopularityProjector likes

[podclaude]

Context

Follow-up from PR #255: Like/Unlike System

Original Finding

The PopularityProjectorTest test helpers in projector_test_case.ex always assume users are public (always add to likes_people). The real PopularityProjector checks UserPrivacy and conditionally adds users.

This means the code path where is_user_public_with_repo? returns false (private user liking a podcast) is untested — likes_people should stay unchanged while likes count still increments.

Additionally, the test helpers duplicate projector logic, creating drift risk if the projector implementation changes.

Acceptance Criteria

• Add test for private user liking a podcast (ID should NOT appear in likes_people)
• Consider refactoring test helpers to call the real projector logic instead of reimplementing it