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/teamNotify.db.js

@@ -211,11 +211,31 @@ async function digestPostsSince(teamId, since, limit = 20) {
   )
 }
 
+/**
+ * The preference rows for a set of users in one Team — the scope-preference
+ * provider's only query (ENGAGEMENT.md Phase 6).
+ *
+ * Returns only the rows that EXIST. Absence is answered by the caller, which is
+ * the same discipline the two recipient queries follow with their COALESCEs: the
+ * default lives in one place and it is the schema.
+ */
+async function prefsForTeam(userIds, teamId) {
+  const ids = userIds.filter(isUserId)
+  if (!ids.length) return []
+  return query(
+    `SELECT user_id, muted, email_mode
+       FROM team_notification_prefs
+      WHERE team_id = ? AND user_id IN (${ids.map(() => '?').join(',')})`,
+    [teamId, ...ids],
+  )
+}
+
 module.exports = {
   recipientIds,
   emailRecipients,
   prefsForUser,
   prefFor,
+  prefsForTeam,
   setPref,
   stampDigest,
   teamsWithForumActivitySince,