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/swagger/swagger-output.json

@@ -13424,6 +13424,67 @@
         }
       }
     },
+    "/api/v1/public/engagement/unsubscribe/{token}": {
+      "post": {
+        "tags": [
+          "Public · Engagement"
+        ],
+        "summary": "Unsubscribe from one channel for one scope",
+        "description": "Honours the tokened link in an engagement email, including RFC 8058 one-click. The token names a delivery channel and a scope; the write turns that channel off for that scope and nothing else. Always answers 200 — a response that distinguished a valid token from a forged one would be an oracle for which (user, scope) pairs exist. Tokens signed before this route existed are still honoured, at this path and at the older /public/teams one.",
+        "parameters": [
+          {
+            "name": "token",
+            "in": "path",
+            "required": true,
+            "schema": {
+              "type": "string"
+            },
+            "description": "The signed token from the email link."
+          }
+        ],
+        "responses": {
+          "200": {
+            "description": "Acknowledged",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/OkFlag"
+                }
+              }
+            }
+          }
+        },
+        "security": [
+          {}
+        ]
+      },
+      "get": {
+        "tags": [
+          "Public · Engagement"
+        ],
+        "summary": "Land a human on the unsubscribe page",
+        "description": "For mail clients that render the List-Unsubscribe URL as an ordinary link. Redirects to the site’s own confirmation page and changes nothing — a GET must not mutate, or a link scanner would unsubscribe people who asked for nothing.",
+        "parameters": [
+          {
+            "name": "token",
+            "in": "path",
+            "required": true,
+            "schema": {
+              "type": "string"
+            },
+            "description": "The signed token from the email link."
+          }
+        ],
+        "responses": {
+          "302": {
+            "description": "Redirect to the site’s unsubscribe page"
+          }
+        },
+        "security": [
+          {}
+        ]
+      }
+    },
     "/api/v1/public/modules": {
       "get": {
         "tags": [
@@ -13811,8 +13872,8 @@
         "tags": [
           "Public · Teams"
         ],
-        "summary": "Unsubscribe from one Team’s notification emails",
-        "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.",
+        "summary": "Unsubscribe from one Team’s notification emails (legacy path)",
+        "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.",
         "parameters": [
           {
             "name": "token",