feat(modules): event triggers, audiences and the ceiling lattice (engagement Phase 2)

The contract half of the engagement system: a module (and core) can DECLARE an
event with a payload contract and fire it. Nothing delivers yet — `emit`
validates, logs and stops, and Phase 4 replaces that log line with the engine.

`api.registerEventTriggers` and `api.registerAudiences` ride the existing
stage()/apply() validate-then-commit discipline, so a registrant that throws
halfway leaves nothing behind. `ctx.events.emit` is fire-and-forget and binds
the owner from the calling module — a module fires its own triggers and no one
else's. `ctx.inbox.push` is present and throws until Phase 7, the shape 1.6.0
settled on for a member that arrives a phase late.

MODULE_API_VERSION 1.7.0 on both halves. Additions only; module-uo's
`coreApi: "^1.3.0"` still resolves.

Three design decisions, approved by the org lead before any code:

ONE NAMESPACE for trigger ids and notification-stream ids (ENGAGEMENT.md §7.2,
against the recommendation in the text). A trigger is a payload contract
attached to an id that may also carry a subscription toggle, so an id has
exactly one owner across both facets, checked in both directions. Core's five
trigger ids ARE its five stream ids, so the same-owner upgrade case is
exercised on every boot rather than only by a module. It keeps
notification_channel_prefs single-keyed in Phase 3, where two namespaces would
have forced a `kind` discriminator into its primary key.

Two knock-on effects appeared only once it was implemented. The id grammar had
to be RELAXED to admit `_` inside a segment — §4.3's own worked example is
`uo.house.idoc_warning`, and two grammars over one namespace would mean an id
legal as a trigger and illegal as the stream it is the same event as. And the
seven grandfathered `uo.*` ids had to share their legacy allowlist with
triggers, because under one namespace `idoc.warning` is a single id. The push
catalog is untouched either way: allStreams() still serves the stream facet
only, so the shipped Android client sees exactly what it saw before.

THE CEILING LATTICE (G24), which the plan named everywhere and defined nowhere.
It is containment, not size: everyone ⊃ authenticated ⊃ {subscribers, members,
staff, owner}, with the four leaves mutually incomparable. The flat total order
the plan's wording invites would let a `staff`-ceilinged trigger be given an
`owner` audience — a rule that mails cheat detection to the player it detected.
Fewer people is not less exposure. Two incomparable ceilings have no meet at
all, so a composition is refused rather than guessed; union-widens is the
intuitive implementation and it is the wrong one.

`kind: 'event' | 'scheduled'` is declarable now and no evaluator exists (§7.1
Q6). Registration accepts `scheduled` and emit refuses to fire one, so `kind`
means something from the moment it can be written rather than from the moment
it is honoured.

Also: `GET /admin/engagement/{triggers,audiences}`, served from the registries
rather than a table so an uninstalled module simply stops appearing;
`npm run engagement:manifest` plus its CI `--check`, the twin of the route
manifest, because renaming a variable breaks stored templates silently, at send
time, in mail someone already received.

Co-Authored-By: Claude <noreply@anthropic.com>

server/src/router/v1/admin/engagement.router.js

@@ -0,0 +1,47 @@
+// Admin · Engagement — the declared event catalog (ENGAGEMENT.md Phase 2).
+//
+// Mounted at /api/v1/admin/engagement by admin/index.js, which has already
+// applied `noindex, isLoggedIn, staffOnly`. Both routes re-gate to `admin`.
+//
+// Admin rather than staff-wide, deliberately. Nothing here is writable yet, but
+// this is the entry point of the screen that decides who receives mail, and the
+// declarations it serves name every variable a template may interpolate. A
+// capability is easier to widen later with a reason than to narrow after an
+// editor has been using it.
+//
+// Rules, templates and the send log arrive under this same prefix in Phases 4
+// and 5, which is why the group exists now with two read routes in it.
+
+const express = require('express')
+
+const controller = require('./engagement.controller')
+const { requireRole } = require('../../../utils/auth')
+
+const engagementRouter = express.Router()
+const adminOnly = requireRole('admin')
+
+engagementRouter.get(
+  '/triggers',
+  // #swagger.tags = ['Admin · Engagement']
+  // #swagger.summary = 'List every declared event trigger, with its payload contract and audience ceiling'
+  // #swagger.description = 'Served from the module registries, not from a table: a trigger is declared in code by core or by an installed module, so this is whatever registered on this boot. Each declaration carries the variables a template may interpolate (with an example per variable, for preview and test-send) and the widest audience a rule may ever give it.'
+  // #swagger.security = [{ "cookieAuth": [] }, { "bearerAuth": [] }]
+  /* #swagger.responses[200] = { description: 'The declared triggers, the audience-ceiling vocabulary, and the variable types', content: { "application/json": { schema: { type: "object", properties: { triggers: { type: "array", items: { type: "object", additionalProperties: true } }, ceilings: { type: "array", items: { type: "object", additionalProperties: true } }, variableTypes: { type: "array", items: { type: "string" } }, kinds: { type: "array", items: { type: "string" } } } } } } } */
+  /* #swagger.responses[403] = { description: 'Not an admin', content: { "application/json": { schema: { $ref: "#/components/schemas/Error" } } } } */
+  adminOnly,
+  controller.listTriggers,
+)
+
+engagementRouter.get(
+  '/audiences',
+  // #swagger.tags = ['Admin · Engagement']
+  // #swagger.summary = 'List every declared audience a rule may be pointed at'
+  // #swagger.description = 'Module-declared named sets of users, resolved over the module own data. The resolver itself is never served — an audience answers with user ids on the server side only.'
+  // #swagger.security = [{ "cookieAuth": [] }, { "bearerAuth": [] }]
+  /* #swagger.responses[200] = { description: 'The declared audiences and the audience-ceiling vocabulary', content: { "application/json": { schema: { type: "object", properties: { audiences: { type: "array", items: { type: "object", additionalProperties: true } }, ceilings: { type: "array", items: { type: "object", additionalProperties: true } } } } } } } */
+  /* #swagger.responses[403] = { description: 'Not an admin', content: { "application/json": { schema: { $ref: "#/components/schemas/Error" } } } } */
+  adminOnly,
+  controller.listAudiences,
+)
+
+module.exports = engagementRouter