Jobs: a background caller can queue a job for a named tenant

Follow-up to #106 (PR #525). A job queued from a place with no request, such as `WorkflowRunner` (a `BackgroundService`), commits on its own in the default tenant, because the provider finds the caller's tenant through the request scope. A background caller in another tenant has no way to say which tenant the job belongs to.

Done when a background caller can queue a job for a named tenant, the record carries that tenant, and the worker runs it in that tenant's session. This blocks moving webhook delivery (#95) onto the queue for any tenant but the default.

Start by tracing WorkflowRunner and the BackgroundService path into the provider that resolves the caller's tenant, then follow the queued job record and worker session setup. Done means a background caller can name a tenant, the record preserves it, and the worker runs the job in that tenant's session; no test file is named in the issue.

Written by the indexing model from the issue text.