Like the client email, the subId is embedded directly in subscription
URLs, so the same characters break it. Validate it on the backend
(Create + Update) and the frontend (Zod), with a localized message
across all 13 locales. An empty subId stays allowed (it is then
auto-generated).
Client emails containing a slash broke the path-param routes
(edit/delete/view returned 404 / "client not found"), leaving stale
records that could only be cleared with manual SQLite edits. Validate
the email on both the backend (Create + Update, which also covers the
bulk paths) and the frontend (Zod) so these characters are rejected at
save time with a clear, localized message across all 13 locales.
Closes#4695
