fix(admin): repair system maintenance controls

Implement server-side searchable user filtering for usage records, including backend search parameters and a shared frontend selector with loading, empty, and pinned-selected states.

Fix announcement deletion by cascading announcement read rows through a Postgres migration and defensive repository cleanup across supported backends.

Wire admin system purge and cleanup endpoints to real data deletion/maintenance flows, improve DataManagement messages, rebuild stats after stats purge, and add runtime OAuth token refresh maintenance when enabled.

Verified with rust-ci equivalent checks: cargo fmt, split clippy, split cargo tests, SQLite/Postgres/MySQL smoke tests, plus frontend npm ci, build, pages build, type-check, and targeted usage selector tests.

apps/aether-gateway/src/handlers/admin/request/system/mod.rs

@@ -60,6 +60,25 @@ impl<'a> AdminAppState<'a> {
         self.app.read_admin_system_stats().await
     }
 
+    pub(crate) async fn purge_admin_system_data(
+        &self,
+        target: aether_data::repository::system::AdminSystemPurgeTarget,
+    ) -> Result<aether_data::repository::system::AdminSystemPurgeSummary, GatewayError> {
+        self.app.purge_admin_system_data(target).await
+    }
+
+    pub(crate) async fn run_admin_system_cleanup_once(
+        &self,
+    ) -> Result<crate::maintenance::AdminSystemCleanupSummary, GatewayError> {
+        self.app.run_admin_system_cleanup_once().await
+    }
+
+    pub(crate) async fn rebuild_admin_stats_once(
+        &self,
+    ) -> Result<crate::maintenance::AdminStatsRebuildSummary, GatewayError> {
+        self.app.rebuild_admin_stats_once().await
+    }
+
     pub(crate) async fn find_proxy_node(
         &self,
         node_id: &str,