Introduces a persisted prompt-template library: migration 012 creates the
templates table (name/description/category_id FK/tags/is_favorite/
template_string), TemplateRepository provides CRUD with partial-update
sentinel semantics, and wp_api/templates.py exposes list/get/create/update/
delete/favorite with FK violations mapped to 400. Bumps the migration-head
version assertions to 12.
