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.

crates/aether-data/src/repository/system.rs

@@ -20,3 +20,28 @@ pub struct AdminSystemStats {
     pub total_api_keys: u64,
     pub total_requests: u64,
 }
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum AdminSystemPurgeTarget {
+    Config,
+    Users,
+    Usage,
+    AuditLogs,
+    RequestBodies,
+    Stats,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, Default, serde::Serialize, serde::Deserialize)]
+pub struct AdminSystemPurgeSummary {
+    pub affected: std::collections::BTreeMap<String, u64>,
+}
+
+impl AdminSystemPurgeSummary {
+    pub fn add(&mut self, key: impl Into<String>, count: u64) {
+        *self.affected.entry(key.into()).or_insert(0) += count;
+    }
+
+    pub fn total(&self) -> u64 {
+        self.affected.values().copied().sum()
+    }
+}