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/teams.router.js

@@ -90,13 +90,20 @@ teamsRouter.get(
   ctrl.getActivity,
 )
 
-// ── One-click unsubscribe (TEAMS.md §6.4) ──────────────────────────────────
+// ── One-click unsubscribe — the LEGACY path (TEAMS.md §6.4) ────────────────
+//
+// The canonical pair now lives at `/public/engagement/unsubscribe/:token`
+// (ENGAGEMENT.md Phase 6). These two stay, permanently, and hand straight to the
+// same handlers: mail sent before that phase carries this path in its
+// `List-Unsubscribe` header, and a route that moves is a person who cannot
+// unsubscribe.
 //
 // Declared last, and the shadowing question is worth answering rather than
 // assuming: these are two segments, so the one-segment '/:slug' cannot take them,
 // and the two-segment '/:slug/members' and '/:slug/activity' both pin a LITERAL
 // second segment. Only a token spelled exactly "members" or "activity" could
-// collide, and a token is `<v>.<uid>.<tid>.<mac>`.
+// collide, and a token is `<v>.<uid>.<tid>.<mac>` (v1) or
+// `<v>.<uid>.<channel>.<scope>.<mac>` (v2).
 //
 // No `siteMode`, unlike every other route in this file. An unsubscribe has to work
 // while the site is in maintenance: the mail that carried the link went out before
@@ -105,8 +112,8 @@ teamsRouter.get(
 teamsRouter.post(
   '/unsubscribe/:token',
   // #swagger.tags = ['Public · Teams']
-  // #swagger.summary = 'Unsubscribe from one Team’s notification emails'
-  // #swagger.description = 'Honours the tokened link in a Team notification email, including RFC 8058 one-click. Sets the same per-Team mute the account screen shows. Always answers 200 — a response that distinguished a valid token from a forged one would be an oracle for which (user, Team) pairs exist.'
+  // #swagger.summary = 'Unsubscribe from one Team’s notification emails (legacy path)'
+  // #swagger.description = 'The pre-Phase-6 path, kept permanently because links in mail already sent point at it. Identical to POST /public/engagement/unsubscribe/{token}. Honours the tokened link including RFC 8058 one-click; a token signed before Phase 6 turns off that Team’s email and no longer mutes its push. Always answers 200 — a response that distinguished a valid token from a forged one would be an oracle for which (user, Team) pairs exist.'
   // #swagger.parameters['token'] = { in: 'path', required: true, schema: { type: 'string' }, description: 'The signed token from the email link.' }
   // #swagger.security = [{}]
   /* #swagger.responses[200] = { description: 'Acknowledged', content: { "application/json": { schema: { $ref: "#/components/schemas/OkFlag" } } } } */