feat(engagement): the email channel on the engine, and the Teams migration (engagement Phase 6)

Email becomes a DeliveryChannel driven by rules, and the Team pipeline stops being
its own thing. `teamNotify.forumPost` now emits an event; a rule decides who is
mailed, through which template, and how often at most. One walk goes forum write
-> events.emit -> rule -> outbox -> worker -> email channel -> template -> SMTP.

Seven decisions settled by the org lead before any code:

  - email only moves; the push tickle and the Discord bridge stay direct calls
  - the EVENT carries its access-checked audience, and `members` resolves to it
  - the four Team rules are seeded DISABLED, with an admin banner and a note
  - team_notification_prefs stays, read by the engine as a scoped preference
  - the payload wins and a structural projection fills the gaps
  - the digest keeps computing at send time; only its state generalizes
  - an unsubscribe token turns off the channel it names, and nothing else

Three defects found while building it:

  - `email.button` never absolutized its href, while image and itemList both
    did. Every rule-driven CTA would have been a dead relative link, because a
    trigger's url variables are validated site-relative by construction.
  - Phase 4a enqueued digest-mode recipients for a drain that Phase 6 decided
    not to build. An outbox row snapshots the payload and so has none of the
    three properties the digest design exists for, including the security one.
  - the digest's send-log row carried no address_hash while the instant row
    beside it did, which would have made half the mail uncorrelatable in Phase 9.

Also: engagement_digest_state + a replay-safe backfill, engagement_outbox.scope_key,
a v2 unsubscribe token that still verifies v1 forever, and the canonical
/public/engagement/unsubscribe pair with the old /public/teams path kept
permanently — mail is not editable once sent.

Verified with 1464 server tests, 324 client tests, and a live rig (MariaDB +
Mailpit + a real Team) covering the instant mail, the digest, the generic
template, a pre-migration unsubscribe link and the backfill's replay-safety.

Docs: RunicGateway/docs#TBD

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

server/src/router/v1/public/index.js

@@ -22,6 +22,7 @@ const wikiRouter = require('./wiki.router')
 const pagesRouter = require('./pages.router')
 const modulesRouter = require('./modules.router')
 const teamsRouter = require('./teams.router')
+const engagementRouter = require('./engagement.router')
 const siteRouter = require('./site.router')
 
 const publicRouter = express.Router()
@@ -41,6 +42,12 @@ publicRouter.use('/modules', modulesRouter)
 // is what populates it (TEAMS.md §10.3). Site-mode gated per route, like the
 // content above it.
 publicRouter.use('/teams', teamsRouter)
+// The unauthenticated half of the engagement system: today exactly the
+// unsubscribe pair. Its own prefix rather than a Teams sub-path, because what a
+// token names is a channel and a scope and a scope is not always a Team
+// (ENGAGEMENT.md Phase 6). Never site-mode gated — an unsubscribe has to work
+// while the site is in maintenance.
+publicRouter.use('/engagement', engagementRouter)
 
 // The four singletons that own no path segment of their own: /settings, /status,
 // /version and /contact. Mounted at the group root, last — safe only because