Add PostgreSQL DurableStore substrate for WP-03

Introduce remora-durable with pool/migrations, a RemoraApiMutation-shaped
ledger, claim-fenced outbox projection, CAS helpers, and keyset cursors,
plus a kind Job acceptance suite against the hermetic fixture.

crates/remora-durable/src/migrate.rs

@@ -0,0 +1,14 @@
+use sqlx::PgPool;
+
+use crate::error::DurableError;
+
+static MIGRATOR: sqlx::migrate::Migrator = sqlx::migrate!("./migrations");
+
+pub async fn migrate(pool: &PgPool) -> Result<(), DurableError> {
+    MIGRATOR.run(pool).await?;
+    Ok(())
+}
+
+pub fn migrator() -> &'static sqlx::migrate::Migrator {
+    &MIGRATOR
+}