Create GitHub Actions workflows for infrastructure and app deployment

## Current Issue

While we expect contributors to this repository to have tested their changes before submitting a Pull Request, and ideally to test after PRs are merged into the `main` branch, there is a persistent challenge: Automated tests are only truly valuable when they run against actual Azure resources. Local unit tests (e.g., Python tests within the GitHub Action runner or Playwright tests with mocked services) add some value, but without live Azure infrastructure, they may miss key integration issues.

## Suggested Solution
- Create `infra-provision.yml` (manual trigger for Bicep deployments)
- Create `app-deploy.yml` (auto-trigger on `app/**`, `static/**`, `templates/**` changes)
- Create `infra-validate.yml` (PR validation for Bicep syntax, auto-trigger on `infra/**` changes)

## Additional information
- GitHub Actions should run against the GitHub Environment called `staging`
- Refer to the pre-existing workflow called `cicd.yml` as a reference implementation