Brings QueueError / PubsubError / InvokeError in line with sibling
shape used by KvError / DocsError / FilesError / SecretsError:
- QueueError gains an explicit Forbidden variant (previously authz
denial was squashed into Rejected("forbidden") in queue_service.rs:68,
losing the structured variant a 403-translation layer would need).
- QueueError::Unavailable renamed → Backend.
- PubsubError::Unavailable renamed → Backend.
- InvokeError::Unavailable renamed → Backend.
- Call sites updated (queue_service, pubsub_service, invoke_service,
Noop* stubs, From<PubsubRepoError> impl, one test assertion).
- New unit test verifying authed-denied → QueueError::Forbidden.
AUDIT.md anchor: F-Q-004.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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