Add optional Shorts sync via derived playlist

[fimenten]

Title: Add optional Shorts sync via derived playlist

Summary:

• Derive the Shorts playlist ID (UUSH...) from UC channel IDs when Include Shorts is enabled.
• Treat Shorts as a playlist source so URL validation for /shorts is unchanged.
• Keep existing channel sync behavior unchanged (default OFF).

Changes:

• Added include_shorts flag and shorts_parent relation on Source.
• Added UC -> UUSH helper to derive the Shorts playlist ID.
• Auto-create/disable the Shorts source when include_shorts toggles.
• UI/Forms updated to expose Include Shorts, plus detail display.
• Migration: 0037_source_include_shorts_and_parent.

Testing:

• docker build: DOCKER_BUILDKIT=1 docker build -t tubesync-test --build-arg FFMPEG_DATE=2025-11-30-14-12 .
• docker test: docker run --rm --entrypoint /usr/bin/python3 tubesync-test /app/manage.py test sync.tests.FrontEndTestCase.test_include_shorts_auto_add_remove
• Manual: verified Shorts playlist fetch and download via TubeSync yt-dlp wrapper.

Notes:

• /shorts tab URL is not directly supported; Shorts are handled as a playlist source.

[fimenten]

Closing for now due to errors; will reopen after fixes.

[tcely]

Please also read #1254 while working on shorts.