Remove unused Trakt collection-template posters and catch orphaned template assets in tests

[Quick104]

Problem

30e9830 removed the six built-in Trakt collection templates but left their poster files behind: 12 files, about 15 MB, in the repository, and the six final JPGs (about 2 MB) still ship in the web build. No test notices, because the asset tests only check that every registered template has its files, not that every file has a template.

The final JPGs may still be in use. A collection created from a template stores the template's poster path (/images/collection-templates/<id>.jpg) and only switches to a stored copy when the upload to artwork storage succeeds. Jellycompat serves those paths straight from the embedded frontend. Existing Trakt collections still sync after the removal, so a Trakt collection created before 30e9830 may still point at one of these JPGs, and deleting them would blank its poster. The raw PNG plates are never served; they only exist to regenerate typography.

I found this by comparing the poster directories with the template IDs in builtin.go while auditing the docs for #1635.

Files with no template:

web/assets-source/collection-templates/raw/trakt_popular_movies.png
web/assets-source/collection-templates/raw/trakt_popular_shows.png
web/assets-source/collection-templates/raw/trakt_recommended_movies.png
web/assets-source/collection-templates/raw/trakt_recommended_shows.png
web/assets-source/collection-templates/raw/trakt_trending_movies.png
web/assets-source/collection-templates/raw/trakt_trending_shows.png
web/public/images/collection-templates/trakt_popular_movies.jpg
web/public/images/collection-templates/trakt_popular_shows.jpg
web/public/images/collection-templates/trakt_recommended_movies.jpg
web/public/images/collection-templates/trakt_recommended_shows.jpg
web/public/images/collection-templates/trakt_trending_movies.jpg
web/public/images/collection-templates/trakt_trending_shows.jpg

Fix

• Delete the six raw PNGs.
• Keep the six JPGs while existing collections can reference them. Remove them later, once stored poster URLs no longer point at them, for example after a migration that copies them into artwork storage.
• Extend the asset tests to fail on any file in either directory without a registered template. Add an explicit allowlist of retired template IDs whose final JPG stays for existing collections, starting with the six Trakt IDs.

Technical notes

• Asset tests: TestBuiltinTemplatePosterAssetsExist (internal/collections/templates/templates_test.go:39) and TestBuiltinTemplateSourcePlatesStayOutOfPublicAssets (templates_test.go:51) iterate List() only.
• Stored poster path: createCollectionFromTemplate sets PosterURL: tmpl.PosterPath (internal/api/handlers/library_collections.go:2119). storeBundledCollectionPosterIfS3Configured (internal/api/handlers/collection_artwork.go:30) replaces it only when the upload succeeds.
• Serving: serveBundledAsset, internal/jellycompat/handlers_images.go:627.
• Legacy Trakt collections keep syncing but can't be changed: internal/apiv2/personal_collections.go:133 and internal/api/handlers/admin_collections_service.go:333.
• .dockerignore already excludes web/assets-source from the image (452990e).
• Some web tests still use Trakt template IDs as fixtures (CollectionTemplateGallery.test.tsx, CollectionTemplateCard.test.tsx). They don't load the assets, so they don't block the cleanup.

AI disclosure

• Harness: T3 Code (Claude Code agent harness)
• Tool(s): Claude Code, GitHub CLI
• Model(s): claude-opus-5-5
• Involvement: AI-assisted; filed at the maintainer's request
• Adversarial review: n/a

[Tobechukwu-Njoku]

I'll take this one, following the fix as written: delete the six raw PNGs, keep the six JPGs behind a retired-template allowlist, and make the asset tests fail on any file without a registered template.