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/lifecycle/bootstrap/postgres.rs

@@ -7,7 +7,7 @@ use tracing::info;
 // Generated by build.rs from schema/bootstrap/postgres.
 pub(crate) static EMPTY_DATABASE_SNAPSHOT_SQL: &str =
     include_str!(concat!(env!("OUT_DIR"), "/empty_database_snapshot.sql"));
-pub(crate) const EMPTY_DATABASE_SNAPSHOT_CUTOFF_VERSION: i64 = 20260505130000;
+pub(crate) const EMPTY_DATABASE_SNAPSHOT_CUTOFF_VERSION: i64 = 20260507000000;
 
 const PUBLIC_BASE_TABLE_COUNT_SQL: &str = r#"
 SELECT COUNT(*)::BIGINT

crates/aether-data/src/lifecycle/migrate/tests.rs

@@ -291,6 +291,7 @@ fn empty_database_snapshot_covers_current_cutoff_versions() {
             20260502000000,
             20260505000000,
             20260505130000,
+            20260507000000,
         ]
     );
 }
@@ -1010,6 +1011,7 @@ fn pending_migrations_from_applied_skips_versions_already_applied() {
             20260502000000,
             20260505000000,
             20260505130000,
+            20260507000000,
         ]
     );
 }