Update how the status of processing services are checked & reported

[coderabbitai]

Context (partial)

For async processing services (distributed workers), the server_live status should be set by the view each time we hear from the processing service. For v1 synchronous processing services with an endpoint_url, those can still be set by the status checker.

Requested Changes (partial)

Rename the following fields on the ProcessingService model:

• last_checked_live → last_seen_live
• last_checked → last_seen

This naming better reflects the semantic difference between:

• Async/pull model: workers report in, and we record when we last saw them
• Sync/push model: we actively check the service status

References

• Requested by: @mihow
• PR: PSV2: endpoint to register pipelines #1076
• Comment: PSV2: endpoint to register pipelines #1076 (comment)