feat(wfe-sqlite): add SQLite persistence provider

SqlitePersistenceProvider using sqlx with in-memory and file-based
SQLite. Schema with 6 tables, 8 indexes, created via ensure_store_exists.
Transactions for multi-table operations. Passes persistence_suite tests.

wfe-sqlite/tests/persistence.rs

@@ -0,0 +1,9 @@
+use wfe_core::persistence_suite;
+
+async fn make_provider() -> wfe_sqlite::SqlitePersistenceProvider {
+    wfe_sqlite::SqlitePersistenceProvider::new(":memory:")
+        .await
+        .unwrap()
+}
+
+persistence_suite!(|| async { make_provider().await });