Avoid double sending of an outbox message

[miaulalala]

Is your feature request related to a problem? Please describe.

The outbox implementation triggers the send from two angles: the REST API call to send and the background job that ensures messages are sent even if the front-end doesn't trigger an API call.

It is conceivable that the two methods could trigger a send at the same time and thus send the outbox message twice. (Possibly even with more errors occurring down the line, for example if an attachment is involved)

Strategies discussed so far:

• locking the table

Describe the solution you'd like

No response

Describe alternatives you've considered

No response

Additional context

No response