Images added from a URL preview and download in the web interface, and other fixes

Fixes

• Previewing or downloading an image added from a URL showed nothing in the web interface. Both now open the image from its own address.
• When sqlite3 wasn't installed on the host, bin/collect_debug.sh put a Python error in its report where the asset count should be. It now reads the database inside the server container, and the report lists each asset with whether Anthias could reach it.

Improvements

• The Anthias website answers "Is Anthias the same as Screenly OSE?", and the page data search engines read no longer wraps titles and answers in extra quote marks.

For contributors

• The settings-save tests write to a temporary config file instead of ~/.anthias/anthias.conf, so they no longer change the defaults that later tests read when both test suites share a data volume.
• CLAUDE.md describes the current layout of the code, and a .claude/skills folder adds notes for AI coding assistants on the boards, the viewer, testing on real hardware and cutting a release.
• Dependabot no longer proposes pillow-heif 1.5 or newer. It can't be built for the 32-bit pi2 and pi3 images against the libheif in Debian Trixie, so pillow-heif stays at 1.4.0.
• The Marketing Screenshots workflow runs from the branch you pick in GitHub when you start it by hand. Its separate ref input is gone.
• Dependencies and GitHub Actions updated.