Email notification customization: internal templates, lifecycle toggles, sender-based wording

Give admins more control over automated emails. Asks include: customize internal notification templates with variables (request title, client name, assignee) — not just client-facing templates; toggle individual lifecycle emails on/off (e.g. subscription cancelled/updated) so clients aren't confused by internal admin adjustments; and differentiate confirmation email wording based on whether the request was created by a client or an admin.