[Future] Add helix-shared-queue-azure-servicebus backend

## Why (not yet — tracking only)

No Azure deployment target exists yet for any Helix service. This issue exists purely to record the design decision for whenever that changes, so it isn't rediscovered from scratch.

## Key decision: target Azure Service Bus, not Azure Storage Queues

SQS FIFO's `MessageGroupId`/`MessageDeduplicationId` (relied on today by the indexer, audit-logger, and media-log producers in helix-api-service) map cleanly onto Azure Service Bus sessions + native duplicate detection. Azure Storage Queues have no ordering or dedup concept at all and would require reimplementing both application-side with weaker guarantees. **Any future Azure backend for `helix-shared-queue` should wrap `@azure/service-bus`, not `@azure/storage-queue`.**

## Decisions deferred to implementation time

- Auth: connection string vs. managed identity (`@azure/identity`/`DefaultAzureCredential`).
- Queue/topic naming and session/dedup-window provisioning (infra-level).
- Message size limits (256KB standard tier matches current `MAX_MESSAGE_SIZE` guards; revisit only if premium tier's 1MB is used).

No code changes proposed by this issue.

Depends on: #1269

This is a tracking issue with no files, tests, or implementation entry point named. Wait for the Azure deployment work and dependency issue #1269, then review the existing helix-shared-queue backends and define the provisioning, authentication, naming, and message-limit decisions before implementation; done is not yet specified.

Written by the indexing model from the issue text.