Add Snyk Elixir security scan workflow

[mikhailbahdashych-glia]

Why

This repository is categorised production_library in DevHub, which requires Snyk coverage. Elixir has no Snyk SCM support — importing through the GitHub Cloud App produces no projects — so the CLI workflow is the only path that scans mix.lock (hex dependencies).

Docs: Snyk for Elixir projects

What this does

• pull_request / merge_group: tests the dependency tree the change would introduce
• push to master: (re-)registers the repo's hex projects in Snyk org glia-devexp (via the SNYK_ORGANIZATION_SLUG Actions variable, already set on this repository)
• weekly schedule + workflow_dispatch: keeps the snapshot fresh when the repository is quiet

Merging this PR is what creates the Snyk projects — the push run performs the snyk monitor registration.

🤖 Generated with Claude Code

[mikhailbahdashych-glia]

⚠️ Heads-up on this merged PR: this repository is public, so the workflow cannot run as merged — public repos cannot call the private salemove/glia-security-workflows reusable workflow, and the org-level SNYK_TOKEN secret is not shared with public repositories. The workflow will show a failed startup on every push to master until fixed. A self-contained follow-up is being prepared; reverting in the meantime is also fine. Apologies for the noise — found by post-creation audit.