Deploy schema to Azure SQL Database from GitHub Actions

This owns the pipeline: the workflow, the login, the firewall and the gate. Not the project (sql-database-projects), the sequencing of a schema change (schema-migrations-safely), the database-side grant (entra-id-auth), or the application (deploy-app-to-azure).

Checked 2026-09-03 against azure/sql-action at tag v2.4, commit eb1f9a80, still the newest tag; SqlPackage 170.4.83.3; sqlcmd 1.10.0; Azure CLI 2.90.0.

The action is three commands. Run them yourself first