feat(data): complete portable SQL backend parity

Align MySQL and SQLite schemas, migrations, usage, stats, export, and backfill behavior with the shared data contracts. Extend gateway startup and maintenance support across all SQL drivers.

apps/aether-gateway/src/data/state/core.rs

@@ -764,7 +764,8 @@ impl GatewayDataState {
     }
 
     pub(crate) fn has_usage_counter_flush_backend(&self) -> bool {
-        self.has_usage_writer() && self.database_driver() == Some(DatabaseDriver::Postgres)
+        self.has_usage_writer()
+            && database_driver_supports_usage_counter_flush(self.database_driver())
     }
 
     pub(crate) fn has_usage_worker_queue(&self) -> bool {
@@ -1075,3 +1076,30 @@ impl GatewayDataState {
         }
     }
 }
+
+fn database_driver_supports_usage_counter_flush(driver: Option<DatabaseDriver>) -> bool {
+    matches!(
+        driver,
+        Some(DatabaseDriver::Postgres | DatabaseDriver::Mysql | DatabaseDriver::Sqlite)
+    )
+}
+
+#[cfg(test)]
+mod usage_counter_flush_backend_tests {
+    use super::database_driver_supports_usage_counter_flush;
+    use aether_data::DatabaseDriver;
+
+    #[test]
+    fn every_sql_driver_supports_usage_counter_flush() {
+        assert!(database_driver_supports_usage_counter_flush(Some(
+            DatabaseDriver::Postgres
+        )));
+        assert!(database_driver_supports_usage_counter_flush(Some(
+            DatabaseDriver::Mysql
+        )));
+        assert!(database_driver_supports_usage_counter_flush(Some(
+            DatabaseDriver::Sqlite
+        )));
+        assert!(!database_driver_supports_usage_counter_flush(None));
+    }
+}