Shared Templates surface — slice 1: email templates

SHIPPED (commit cb3db4e): one admin Templates surface for the whole platform (ADR 0036) instead of per-module template builds. Slice 1 delivered the email template kind with all 21 platform-default email templates authored and one real Novu send proven. Closes the long-standing "admin templates: Not built" gap in launch-scope item 8. Slices 2 (document/engagement-letter kind) and 3 (form kind) are Planned cards on this roadmap.