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/test/engagementEngine.test.js

@@ -475,10 +475,11 @@ test('two sweepers racing one due row: exactly one claim wins', async () => {
 // ── The send log ───────────────────────────────────────────────────────────
 
 test('a row whose channel has no deliver() finishes failed, and the send log says why', async () => {
-  // Phase 4a delivers nothing: `deliver` arrives with email in Phase 6 and the
-  // inbox in Phase 7. Recording 'sent' would be a lie in the one table whose
-  // purpose is answering "did they get it".
-  addRule()
+  // `inapp`, because as of Phase 6 `email` DOES deliver. The inbox arrives in
+  // Phase 7, and until then recording 'sent' would be a lie in the one table
+  // whose purpose is answering "did they get it".
+  addRule({ channels: ['inapp'] })
+  optIn(10, 'uo.house.idoc_warning', 'inapp')
   await engine.dispatch(event(), T0)
   await worker.tick(later(1000))
 
@@ -545,14 +546,20 @@ test('absence means the CHANNEL default, and all three of core default off', asy
   assert.equal(channels.defaultMode('email'), 'off')
 })
 
-test("a 'digest' preference still enqueues — batching is the drain's job, not the enqueue's", async () => {
+// **This reverses what Phase 4a asserted here**, and the reversal is Phase 6's
+// §4.2b decision rather than a change of mind about queues. A digest is
+// re-derived from the source tables at send time — that is what makes a hidden
+// post absent from it and a user who lost access unreachable by it — so an outbox
+// row for a digest recipient would be a second copy of the content with none of
+// those properties. Nothing drains it, so nothing writes it.
+test("a 'digest' preference does NOT enqueue — the digest re-derives at send time", async () => {
   registries._reset()
   registerUoTrigger({ ceiling: 'authenticated', audience: 'authenticated' })
   optIn(10, 'uo.house.idoc_warning', 'email', 'digest')
   addRule({ audience: 'authenticated' })
 
   await engine.dispatch(event(), T0)
-  assert.equal(outboxRows().length, 1)
+  assert.equal(outboxRows().length, 0)
 })
 
 // ── The hourly ceiling (§7.1 Q3) ───────────────────────────────────────────