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/tx.rs

@@ -0,0 +1,42 @@
+use std::future::Future;
+use std::pin::Pin;
+
+use sqlx::{PgPool, Postgres, Transaction};
+
+use crate::error::DurableError;
+
+pub type PgTx<'a> = Transaction<'a, Postgres>;
+pub type BoxFuture<'a, T> = Pin<Box<dyn Future<Output = T> + Send + 'a>>;
+
+pub async fn with_transaction<T, F>(pool: &PgPool, work: F) -> Result<T, DurableError>
+where
+    F: for<'c> FnOnce(&'c mut PgTx<'_>) -> BoxFuture<'c, Result<T, DurableError>>,
+{
+    let mut tx = pool.begin().await?;
+    match work(&mut tx).await {
+        Ok(value) => {
+            tx.commit().await?;
+            Ok(value)
+        }
+        Err(error) => {
+            let _ = tx.rollback().await;
+            Err(error)
+        }
+    }
+}
+
+/// Bound how long this transaction waits for row or advisory locks.
+pub async fn set_lock_timeout(tx: &mut PgTx<'_>) -> Result<(), DurableError> {
+    sqlx::query("SET LOCAL lock_timeout = '5s'")
+        .execute(tx.as_mut())
+        .await?;
+    Ok(())
+}
+
+pub async fn advisory_xact_lock(tx: &mut PgTx<'_>, key: i64) -> Result<(), DurableError> {
+    sqlx::query("SELECT pg_advisory_xact_lock($1)")
+        .bind(key)
+        .execute(tx.as_mut())
+        .await?;
+    Ok(())
+}