Less black screen when a video gives way to an image, and other fixes

Fixes

• Add asset opens with an empty form every time. The URL of the last web asset you added no longer sits in the box, one press of Enter away from a duplicate, and the Apps tab starts from the list of apps instead of the last app's form.
• The Actions column in the asset table fits its buttons, so the icons no longer spill past the edge of the table.
• Going from a video or a web page to an image shows less black screen. While the image loads, the screen holds the last image Anthias showed instead of going black.
• On Docker Compose installs of boards other than the Raspberry Pi, System info shows the board's name, such as FriendlyElec NanoPi R3S LTS, instead of "Generic aarch64 Device". The device_model field of /api/v2/info, which was empty on these boards, now has the name too.
• When Anthias can't identify the board and so can't check a video's codec, the upload message says what to check: on Docker Compose installs, that anthias-host-agent and Redis are running. It used to tell you to re-flash with a board-specific image that doesn't exist.
• Installing on Raspberry Pi OS Trixie no longer fills the output with Ansible deprecation warnings.

For contributors

• The Ansible playbook reads gathered facts through ansible_facts, and the new ansible/ansible.cfg sets inject_facts_as_vars = False, so a bare fact reference fails instead of slipping back in. That file takes precedence over /etc/ansible/ansible.cfg when the installer runs the playbook.
• New tests cover chunked uploads and their error messages, page-wide drag and drop, and the Add asset reset. A fake docker command tests the image cleanup, and an Ansible test runs the watchdog selection against made-up sysfs trees, which is how the x86 case is covered.
• The repository has an MJML newsletter template in emails/, with a CI job that validates it and checks the compiled size against Gmail's clipping limit.
• The Anthias website adds pages on free digital signage software and on Yodeck and piSignage, expands the Raspberry Pi pages, and links to the Screenly shop for buying a player.

Django moves from 5.2.14 to 5.2.17, which brings its upstream security fixes, along with the usual Python, JavaScript and GitHub Actions dependency updates.