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/model/teams/teamSync.model.js

@@ -217,8 +217,23 @@ async function notifyRoster(team, { joined, promoted, demoted }) {
     // The count rides along for the Discord bridge (§7.2), which has no app on
     // the other end to pull the roster after a content-free nudge. The tickle
     // itself is unchanged and still carries nothing.
-    if (joined.length > 0) await teamNotify.memberJoined(team, { count: joined.length })
-    if (promoted.length > 0 || demoted.length > 0) await teamNotify.leadershipChanged(team)
+    // `names` is the engagement engine's half (ENGAGEMENT.md Phase 6): the two
+    // triggers declare `memberName` / `leaderName` as required single values, so
+    // the fan-out emits one event per person while the tickle and the bridge stay
+    // one per run. A member the module reported without a display name is skipped
+    // rather than emitted as "someone" — a required variable filled with a
+    // placeholder is a mail that names nobody.
+    if (joined.length > 0) {
+      await teamNotify.memberJoined(team, {
+        count: joined.length,
+        names: joined.map((m) => m.display_name).filter(Boolean),
+      })
+    }
+    if (promoted.length > 0 || demoted.length > 0) {
+      await teamNotify.leadershipChanged(team, {
+        names: promoted.map((m) => m.display_name).filter(Boolean),
+      })
+    }
   } catch (err) {
     log.warn('roster notification not sent', { teamId: team.id, message: err.message })
   }