Tailor signature certificate email copy for approvals

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>

pkg/coredata/migrations/20260515T140410Z.sql

@@ -0,0 +1,19 @@
+-- Copyright (c) 2026 Probo Inc <hello@getprobo.com>.
+--
+-- Permission to use, copy, modify, and/or distribute this software for any
+-- purpose with or without fee is hereby granted, provided that the above
+-- copyright notice and this permission notice appear in all copies.
+--
+-- THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+-- REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+-- AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+-- INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+-- LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+-- OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+-- PERFORMANCE OF THIS SOFTWARE.
+
+ALTER TABLE electronic_signatures
+    ADD COLUMN email_subject TEXT NOT NULL DEFAULT '';
+
+ALTER TABLE electronic_signatures
+    ALTER COLUMN email_subject DROP DEFAULT;