Regenerate submitter slug when recipient email/phone is changed

[StevieDrew]

Problem

When a submitter's email or phone is changed (reassigning a document to a different recipient), the signing slug
remains the same. This means the old recipient can still access the signing page via the old URL (/s/{slug}),
which is a security concern.

Expected Behavior

When a submitter's email or phone is updated from a previously set value, the slug should be regenerated so the old
signing link is automatically invalidated.

Proposed Solution

Add a before_save callback on the Submitter model that regenerates the slug when:

• Email changes from a non-blank value (reassignment, not initial assignment)
• Phone changes from a non-blank value

This approach centralizes the logic in the model so it works across all code paths (web UI, API, console) without
modifying any controllers.