fix(audit): coalesce identical plugin routine audits so they cannot flush the log (#1667)

Successful plugin_as_user and plugin_service_* entries written by a plugin
with the same user, action, resource and details are recorded once per
10 minutes. Failures and every other action are still written each time.

src/backend/tests/plugins/security/ctx-guards.test.ts

@@ -75,7 +75,8 @@ const UNGATED: Record<string, string> = {
   "schedule.every": "the plugin's own timers",
   "schedule.after": "the plugin's own timers",
   "audit.record": "writes under the runtime's actor, never a plugin value",
-  asUser: "always audited as plugin_as_user",
+  asUser:
+    "audited as plugin_as_user (identical successful repeats coalesced for 10 minutes)",
   currentActor: "reads the actor, cannot set it",
   "desktop.available": "a yes/no about the environment",
   "http.baseUrl": "reads the request's public origin",