Generate thumbnails of subscribed playlists (related: #4015)

[shiny-comic]

The bug is described in #4015.
This PR enables the generation of thumbnails for newly subscribed playlists.

Note: This fix DOES NOT regenerate thumbnails of existing subscribed playlists

[Copilot]

Pull request overview

Fixes #4015 by persisting a thumbnail reference when a user subscribes to a YouTube playlist, so “Saved playlists” can render a non-gray thumbnail for newly subscribed playlists.

Changes:

• Persist thumbnail_id on subscribed playlists at creation time.
• Add thumbnail_id column to the playlists table (SQL schema + DB migration).
• Remove DB ignore annotation so thumbnail_id is serialized to/from the database.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/invidious/playlists.cr	Stores a derived thumbnail_id when subscribing; InvidiousPlaylist now persists this field.
src/invidious/database/migrations/0011_add_thumnail_id_to_playlists.cr	Adds a migration to append thumbnail_id to public.playlists.
config/sql/playlists.sql	Updates canonical schema to include thumbnail_id.

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[unixfox]

Thank you. Do note that for the moment unfortunately all PR with SQL migrations are blocked until we can figure out a way to handle them properly.

[shiny-comic]

@unixfox thanks for the reply. Should I close this or keep it open?

[unixfox]

you can keep it opened, we plan on fixing the sql migrations one day

[shiny-comic]

@unixfox OK thanks.