feat: add candidate declined pipeline status

[anonymoususer72041]

Summary

Adds a new candidate-joborder pipeline status, Candidate Declined, positioned between Not in Consideration and Client Declined.

The new status is added to the installation schema and to the upgrade migration for existing installations. It is also included in pipeline statistics, graphs and email notification settings.

Email notification settings now handle Candidate Declined separately from Client Declined. Existing installations are handled defensively when their serialized status email settings do not yet contain the new status ID, falling back to the database default instead of producing missing-key notices.

Motivation

The new status provides a clearer distinction between cases where a candidate is not submitted to a client, cases where the client rejects a submitted candidate and cases where the candidate declines.

Not in Consideration is better suited for candidates that are not presented to the client at all. Client Declined indicates a client-side rejection after the candidate has been submitted or considered by the client. However, there was no dedicated status for cases where the candidate declines from their side, for example after an interview.

Adding Candidate Declined makes it possible to represent these candidate-side rejection scenarios more accurately in the pipeline.