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/notificationChannelPrefs/notificationChannelPrefs.db.js

@@ -41,4 +41,27 @@ async function offPushExcept(userId, keep) {
   )
 }
 
-module.exports = { listByUser, upsert, offPushExcept }
+/**
+ * Turn one channel off for every named stream — the deployment-wide unsubscribe
+ * (ENGAGEMENT.md Phase 6).
+ *
+ * It WRITES a row per stream rather than updating the rows that happen to exist,
+ * and the difference is the same one `offPushExcept` argues: absence means the
+ * channel's `defaultMode`, so updating only what is there would leave a user
+ * unsubscribed today and re-subscribed the day a channel ships a non-off default.
+ * An unsubscribe has to be a statement, not the absence of one.
+ */
+async function offForChannel(userId, channel, streamIds) {
+  const ids = [...new Set(streamIds)].filter((s) => typeof s === 'string' && s)
+  if (!ids.length) return null
+  const values = ids.map(() => '(?, ?, ?, ?)').join(', ')
+  const params = ids.flatMap((streamId) => [userId, streamId, channel, 'off'])
+  return query(
+    `INSERT INTO notification_channel_prefs (user_id, stream_id, channel, mode)
+     VALUES ${values}
+     ON DUPLICATE KEY UPDATE mode = VALUES(mode)`,
+    params,
+  )
+}
+
+module.exports = { listByUser, upsert, offPushExcept, offForChannel }