ci: #3631526 Add GitLab CI and README

Adds GitLab CI and the README for #3631526. Issue: https://www.drupal.org/project/ai_media/issues/3631526

• .gitlab-ci.yml on drupal/gitlab_templates, pinned to 1.17.3, current Drupal 11 on PHP 8.4. Every validate job gates the merge (composer-lint, phpcs, phpstan, cspell, eslint, stylelint). PHPUnit and Nightwatch are skipped until the module has tests.
• README matches the project page and says git.drupalcode.org is canonical, GitHub a mirror.
• Small fixes so the jobs pass: one phpcs indentation, US spelling, Prettier and property order in the JS and CSS, a project word list for cspell.
• enshrined/svg-sanitize moves from >=0.15 <1.0 to ^0.22 || ^1.0: every release up to 0.22.0 has advisories, so Composer refuses to install them.

AI-Generated: Yes

Checkpoints:

• File an issue
• Addition/Change/Update/Fix
• Testing to ensure no regression
• Automated unit testing coverage
• Automated functional testing coverage
• UX/UI designer responsibilities
• Readability
• Accessibility
• Performance
• Security
• Developer Documentation
• User Guide Documentation
• Reviewed by human
• Code review by maintainers
• Full testing and approval
• Credit contributors
• Review with the product owner
• Release notes snippet
• Release