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/routes.guards.json

@@ -167,6 +167,15 @@
         "validate"
       ]
     },
+    {
+      "method": "GET",
+      "path": "/api/v1/admin/engagement/audience-preview",
+      "handlers": 2,
+      "gates": [
+        "noindex",
+        "requireAuth"
+      ]
+    },
     {
       "method": "GET",
       "path": "/api/v1/admin/engagement/audiences",
@@ -176,6 +185,105 @@
         "requireAuth"
       ]
     },
+    {
+      "method": "GET",
+      "path": "/api/v1/admin/engagement/channels",
+      "handlers": 2,
+      "gates": [
+        "noindex",
+        "requireAuth"
+      ]
+    },
+    {
+      "method": "GET",
+      "path": "/api/v1/admin/engagement/rules",
+      "handlers": 2,
+      "gates": [
+        "noindex",
+        "requireAuth"
+      ]
+    },
+    {
+      "method": "POST",
+      "path": "/api/v1/admin/engagement/rules",
+      "handlers": 2,
+      "gates": [
+        "noindex",
+        "requireAuth"
+      ]
+    },
+    {
+      "method": "DELETE",
+      "path": "/api/v1/admin/engagement/rules/:id",
+      "handlers": 2,
+      "gates": [
+        "noindex",
+        "requireAuth"
+      ]
+    },
+    {
+      "method": "GET",
+      "path": "/api/v1/admin/engagement/rules/:id",
+      "handlers": 2,
+      "gates": [
+        "noindex",
+        "requireAuth"
+      ]
+    },
+    {
+      "method": "PUT",
+      "path": "/api/v1/admin/engagement/rules/:id",
+      "handlers": 2,
+      "gates": [
+        "noindex",
+        "requireAuth"
+      ]
+    },
+    {
+      "method": "PATCH",
+      "path": "/api/v1/admin/engagement/rules/:id/enabled",
+      "handlers": 2,
+      "gates": [
+        "noindex",
+        "requireAuth"
+      ]
+    },
+    {
+      "method": "GET",
+      "path": "/api/v1/admin/engagement/segments",
+      "handlers": 2,
+      "gates": [
+        "noindex",
+        "requireAuth"
+      ]
+    },
+    {
+      "method": "POST",
+      "path": "/api/v1/admin/engagement/segments",
+      "handlers": 2,
+      "gates": [
+        "noindex",
+        "requireAuth"
+      ]
+    },
+    {
+      "method": "DELETE",
+      "path": "/api/v1/admin/engagement/segments/:id",
+      "handlers": 2,
+      "gates": [
+        "noindex",
+        "requireAuth"
+      ]
+    },
+    {
+      "method": "PUT",
+      "path": "/api/v1/admin/engagement/segments/:id",
+      "handlers": 2,
+      "gates": [
+        "noindex",
+        "requireAuth"
+      ]
+    },
     {
       "method": "GET",
       "path": "/api/v1/admin/engagement/triggers",