feat(notificaciones): edit summary recipients in the UI

NOTIFICATION_ADMIN_EMAILS made "add Beto to the summaries" a redeploy —
the wrong unit of work for a list that changes when office staff change.

Adds `app_settings`, a key/value table for the configuration staff must
be able to change without a deploy, and `SettingsService`, which resolves
every key db -> env -> default and reports which of the three a value
came from. That ladder is what makes the move safe: a deployment behaves
exactly as before until somebody saves in the UI, and the screen can say
"this is still coming from the deployment" rather than implying somebody
chose it.

- new ability `setting:manage` (ADMIN) — deliberately above
  `notification:send`, since redirecting the audit summaries is how
  someone would quietly stop them being read
- GET/PUT /notifications/settings/admin-emails; read is open to any
  logged-in user so the UI can display the list, write is gated
- resolved per job, not cached at boot, or we would reintroduce exactly
  the restart-to-apply behaviour being removed
- a saved empty list means "nobody" and does NOT fall through to the env,
  or clearing the field would keep mailing the people just removed

Credentials stay in env — see the model doc for where the line is drawn.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

apps/api/src/settings/settings.module.ts

@@ -0,0 +1,14 @@
+import { Module } from "@nestjs/common";
+import { SettingsService } from "./settings.service";
+
+/**
+ * Operator-editable configuration. No controller of its own — each setting is
+ * exposed by the feature that owns it (summary recipients live under
+ * /notifications), so the validation and the permission live next to the
+ * thing they protect rather than behind a generic key/value endpoint.
+ */
+@Module({
+  providers: [SettingsService],
+  exports: [SettingsService],
+})
+export class SettingsModule {}