Add queue manager tests and youtube testing mocks

[nextinfinity]

Summary

• add a configurable video fetcher abstraction with a test mock implementation
• add queue manager tests for tick-based progression and vote-skip behavior
• update queue progression tests to avoid wall-clock waits and use the new tick path

Verification

• mix test test/streamshore/queue_manager_test.exs test/streamshore_web/controllers/video_controller_test.exs test/streamshore_web/controllers/playlist_controller_test.exs