feat(wfe-postgres): add PostgreSQL persistence provider

PostgresPersistenceProvider using sqlx with wfc schema. JSONB for
complex fields, TIMESTAMPTZ for dates. Transactions for atomicity.
Proper error propagation on deserialization (no unwrap_or_default).
Status conversion returns Result for unknown values.

wfe-postgres/tests/persistence.rs

@@ -0,0 +1,16 @@
+use wfe_core::persistence_suite;
+use wfe_core::traits::PersistenceProvider;
+
+async fn make_provider() -> wfe_postgres::PostgresPersistenceProvider {
+    let database_url = "postgres://wfe:wfe@localhost:5433/wfe_test";
+
+    let provider = wfe_postgres::PostgresPersistenceProvider::new(database_url)
+        .await
+        .expect("Failed to connect to PostgreSQL. Is the database running?");
+
+    provider.ensure_store_exists().await.unwrap();
+    provider.truncate_all().await.unwrap();
+    provider
+}
+
+persistence_suite!(make_provider);