feat(engagement): templates — the email block family, renderer and seeded set (engagement Phase 5a)

Every subject and body moves out of `mailer.js` into `engagement_templates` rows an
operator can edit. A relocation, not a regression: nothing that sends mail today
starts depending on an operator authoring something first.

- `email.*` block family in its own registry, sharing the page family's envelope
  walk and validate-then-sanitize order by binding rather than by copy.
- A server-side renderer producing both parts of a multipart message; the text
  part is byte-identical to the literals this commit deletes.
- Nine seeded templates, six of them wired now; the seeder's `customized = 0`
  guard lives in the UPDATE's own WHERE.
- `renderByKey` falls back to the shipped seed when a row is missing or unusable,
  so no failure of the table can stop a password reset.

Also fixes `check:hosts` reading the template key `auth.email-verify` as the
hostname `auth.email`.

Co-Authored-By: Claude <noreply@anthropic.com>

server/db/seed.js

@@ -4,6 +4,7 @@ const settingsDb = require('../src/model/settings/settings.db')
 const wikiDb = require('../src/model/wiki/wiki.db')
 const users = require('../src/model/users/users.model')
 const { ensureSchema, close } = require('../src/utils/db')
+const { seedTemplates } = require('../src/engagement/templates')
 const brand = require('../src/config/brand')
 
 const log = require('../src/utils/logger')('seed')
@@ -74,6 +75,12 @@ async function seedDefaults() {
     // migration of pages seeded before the wiki upgrade).
     await wikiDb.assignCategoryBySlug(slug, categorySlug)
   }
+  // The shipped mail bodies (ENGAGEMENT.md §4.6.1). Idempotent, and it never
+  // overwrites a row an operator has edited — `customized = 1` is checked in the
+  // UPDATE's own WHERE, not in a read-then-write. Never throws: a template that
+  // failed to seed costs the shipped default, which `renderByKey` falls back to
+  // anyway, and must not stop a boot.
+  await seedTemplates()
   log.info('settings and wiki defaults ensured')
 }