Store the per-signature email subject as text on the electronic_signatures
row at creation time, mirroring the consent_text pattern. The document
approval service sets "Your approved <Title> - Certificate of Completion";
other callers default to "Your signed <Name> - Certificate of Completion".
The certificate worker uses signature.email_subject as the email subject,
falling back to the default format when the column is empty.
Signed-off-by: Sacha Al Himdani <sacha@getprobo.com>
6948a6b9ab