Search backend healthcheck process

As we have a new search backend coming in !4109 (merged) the complexity of the infrastructure is growing. There is no point in attempting to process search indexing jobs in the Oban queue if the backend is not healthy, so we can monitor it easily with a GenServer.

Healthcheck reporting is available for Meilisearch as well as the coming Qdrant+FastEmbed integration. This will also prevent an outage of a search backend service to cause us to completely lose search indexing jobs after the Oban job attempts have been exhausted.

This change also starts the search indexing queue as paused until the first check has been completed.