Complete the switch to Azure Service Bus

[SandGrainOne]

Description

The work we did to replace Storage Queues with Azure Service Bus (ASB) was interrupted because it included replacing Azure Functions with in-process command handlers in Events using Wolverine. We ran into the problem that the Events API's outbound IP differs from the Functions app's outbound IP, which would break subscribers that firewall on IP — both for outbound event delivery and for subscription-validation webhook calls.

This issue tracks completing the ASB transition while keeping subscriber-facing webhook calls (outbound delivery, subscription validation) on Azure Functions until subscribers can whitelist the main Events API's IP instead. Registration and inbound have no subscriber-facing HTTP call and move fully in-process.

Not in scope here (tracked separately, later)

• Removing drained Storage Queues and the corresponding legacy Events.Functions once their queues are fully drained.
• Retiring the outbound and subscription-validation Functions for good, moving those two paths back to in-process handlers the same way registration/inbound already work. Tracked as Route outbound and subscription-validation to in-process handlers once DIS IP is whitelisted #1076 — blocked on the separate DIS hosting migration.
• Remove deployment pipelines for the now-dead Events.Functions project once it's retired.