Set up Azurite-based testing infrastructure for integration tests

Goal

Provide a local Azure Storage emulator (Azurite) so integration tests can exercise real blob/container operations without hitting a real Azure subscription.

Why

We need confidence in storage operations before building any UI. Testing against a live Azure account is slow, costly, and not reproducible in CI; Azurite gives us a fast, free, deterministic emulator.

Tasks

• Add Azurite as a dev dependency (Docker image, or Testcontainers for .NET — pick whichever is simplest to run locally and in CI).
• Document how to start Azurite locally in README.md (or a new docs/testing.md).
• Wire Azurite startup/shutdown into AzStorage.IntegrationTests (e.g. via a shared IAsyncLifetime fixture) so tests are self-contained.
• Add the Azurite step to the GitHub Actions workflow (depends on the CI issue) so integration tests run in CI too.

Acceptance Criteria

• A sample integration test can connect to the emulator, create a container, and list it.
• Integration tests run both locally (dotnet test) and in CI without any real Azure credentials.

Start by inspecting the AzStorage.IntegrationTests project and the existing GitHub Actions workflow, then choose either a Docker-based Azurite service or Testcontainers for .NET. Check README.md or docs/testing.md for the local setup instructions. Done means a sample test creates and lists a container against Azurite, and dotnet test works locally and in CI without Azure credentials.