feat(engagement): Admin - Engagement - Rules and Audiences (engagement Phase 4b)

The admin surface over the Phase 4a engine: two screens, twelve routes and the
reach preview. Nothing in the engine changed; what changed is that an operator
can now reach it.

Four decisions settled by the org lead before any code:

  - segments get their OWN nav entry, "Audiences", not a tab of the rules screen
  - the on/off switch is its own PATCH route, not a full PUT
  - the reach preview is a count only, on demand
  - a rule can be hard-deleted; the send log survives it

The switch is the one with real content in it. A PUT re-validates against the
registries as they are NOW, so the rules a re-validating toggle cannot switch
off are exactly the three an operator most wants stopped: a rule whose module
was uninstalled, one naming a channel that is gone, and one whose trigger has
since narrowed its ceiling under a saved audience. PATCH .../enabled writes one
column and always works. Switching ON unvalidated is safe because the engine
re-checks the ceiling at send time.

The preview calls the engine's own resolver rather than a second query that
agrees with it today, and answers a count and nothing else - the resolver's
output for a module-declared segment is a set of players derived from game data.
It reports `capped` at the 5000-row bound (the count is a floor, not a total),
`reason` for an `owner` audience (which resolves per event and has no advance
answer), and `permitted` so the editor cannot show a healthy number beside a
save the server will refuse.

Two defects found by walking it against a live server, both in Phase 4a's code:

  1. A rule pointing at a DORMANT segment read as healthy. listAnnotated asked
     only whether the segment ROW existed. The other shape of the same failure
     is a segment sitting exactly where it was whose every audience belongs to
     an uninstalled module: same outcome, nothing deleted. Uninstalling a module
     under an enabled rule produced a rule the screen showed as on and firing.
     The expression walk now lives in engagement/segments.js as
     `missingAudiences` and both lists ask it.
  2. "1 rule still use this segment" - the delete refusal pluralised the noun
     and not the verb, in the sentence an operator reads when told no.

Also: a rule's trigger is now a stated rule rather than an omission in the
UPDATE statement (its cooldowns, queued sends and history are all about one
trigger id); a condition tree the editor cannot render is shown read-only rather
than flattened, because flattening changes which events fire the rule; and
literals are coerced client-side to the type the trigger declared, with anything
that does not parse passed through unchanged so the server's refusal names the
variable.

Tests: 21 new server tests (test/engagementAdmin.test.js) and 25 client ones
(client/test/engagementRules.test.js), all green. The single failure in the
server suite (`the committed manifest matches the declarations in the tree`) is
the known Windows CRLF artifact and fails identically on clean edge.

Companion docs PR: docs#184.

- [x] AI-assisted: written with Claude Code (Opus)

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

server/src/model/engagement/engagementRules.model.js

@@ -23,6 +23,7 @@ const segmentsDb = require('./engagementSegments.db')
 const registries = require('../../modules/registries')
 const ceilings = require('../../modules/ceilings')
 const channels = require('../../engagement/channels')
+const segmentExpressions = require('../../engagement/segments')
 const conditions = require('../../engagement/conditions')
 
 // A day. Longer than this and "cooldown" is really "send once", which a rule
@@ -192,6 +193,48 @@ async function update(id, input) {
   return { ok: true, rule: await db.getById(id) }
 }
 
+/**
+ * Why this rule cannot currently fire, as a list of sentences. Empty = it can.
+ *
+ * **Three ways, not two.** A rule can be dormant because its trigger is gone,
+ * because a channel it names is gone, or because its AUDIENCE is gone - and the
+ * audience case has two shapes that a screen must not collapse into one:
+ *
+ *   • the segment row was deleted out from under it (§7.3), or
+ *   • the segment still exists and every audience in it belongs to a module that
+ *     has been uninstalled (§5.1a rule 4).
+ *
+ * Both leave the rule reaching nobody. Only the first leaves nothing behind, and
+ * a check that asks only "does the row exist" reports the first and misses the
+ * second - which shows an enabled, healthy-looking rule that cannot fire. Found
+ * by uninstalling a module under a live rule while building Phase 4b's screen.
+ *
+ * @param {Map<number, {expression: object}>} segments  every segment, by id
+ */
+function dormancyReasons(rule, segments) {
+  const reasons = []
+  if (!registries.eventTrigger(rule.trigger_id)) reasons.push(`trigger "${rule.trigger_id}" is not registered`)
+  if (rule.audience_segment_id) {
+    const segment = segments.get(rule.audience_segment_id)
+    if (!segment) reasons.push('its audience segment no longer exists')
+    else {
+      const missing = segmentExpressions.missingAudiences(segment.expression)
+      if (missing.length) {
+        reasons.push(`its audience "${segment.name}" uses ${missing.join(', ')}, which nothing registers`)
+      }
+    }
+  }
+  for (const c of rule.channels || []) if (!channels.has(c)) reasons.push(`channel "${c}" is not registered`)
+  return reasons
+}
+
+const annotate = (rule, segments) => {
+  const reasons = dormancyReasons(rule, segments)
+  return { ...rule, dormant: reasons.length > 0, dormantReasons: reasons }
+}
+
+const segmentsById = async () => new Map((await segmentsDb.list()).map((s) => [s.id, s]))
+
 /**
  * List every rule, each annotated with whether it can currently fire.
  *
@@ -202,23 +245,59 @@ async function update(id, input) {
  */
 async function listAnnotated() {
   const rows = await db.list()
-  const segments = new Map((await segmentsDb.list()).map((s) => [s.id, s]))
-  return rows.map((rule) => {
-    const reasons = []
-    if (!registries.eventTrigger(rule.trigger_id)) reasons.push(`trigger "${rule.trigger_id}" is not registered`)
-    if (rule.audience_segment_id && !segments.has(rule.audience_segment_id)) {
-      reasons.push('its audience segment no longer exists')
-    }
-    for (const c of rule.channels || []) if (!channels.has(c)) reasons.push(`channel "${c}" is not registered`)
-    return { ...rule, dormant: reasons.length > 0, dormantReasons: reasons }
-  })
+  const segments = await segmentsById()
+  return rows.map((rule) => annotate(rule, segments))
+}
+
+/** One rule with the same dormancy annotation the list carries, or null. */
+async function getAnnotated(id) {
+  const rule = await db.getById(id)
+  if (!rule) return null
+  return annotate(rule, await segmentsById())
+}
+
+/**
+ * Turn one rule on or off, writing that column and no other (Phase 4b).
+ *
+ * This is the one write path that does NOT go through `validate`, and the
+ * asymmetry is deliberate. Switching a rule OFF must always be possible - a rule
+ * whose module has been uninstalled, or whose trigger has since narrowed its
+ * ceiling under a saved audience, is exactly the rule an operator most urgently
+ * wants stopped, and it is exactly the rule `validate` would now refuse. The
+ * full editor still re-validates on save, and the engine re-checks the ceiling at
+ * send time, so nothing is loosened by having a switch that is only a switch.
+ */
+async function setEnabled(id, enabled, updatedBy = null) {
+  const existing = await db.getById(id)
+  if (!existing) return { ok: false, errors: [`no rule ${id} exists`], notFound: true }
+  await db.setEnabled(id, enabled, updatedBy)
+  return { ok: true, rule: await getAnnotated(id) }
+}
+
+/**
+ * Delete a rule.
+ *
+ * Its cooldown rows and any still-pending outbox rows go with it (both carry an
+ * ON DELETE CASCADE), and that is the right blast radius: neither means anything
+ * without the rule. `engagement_sends` deliberately does NOT — its `rule_id`
+ * carries no foreign key — so the send log outlives the rule and the record of
+ * what was actually mailed survives an operator tidying up.
+ */
+async function remove(id) {
+  const existing = await db.getById(id)
+  if (!existing) return { ok: false, errors: [`no rule ${id} exists`], notFound: true }
+  await db.remove(id)
+  return { ok: true }
 }
 
 module.exports = {
   validate,
   create,
   update,
+  setEnabled,
+  remove,
   listAnnotated,
+  getAnnotated,
   MAX_COOLDOWN_SECONDS,
   MAX_DELAY_SECONDS,
   MAX_SENDS_PER_HOUR,