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-domain/src/fingerprint.rs

@@ -7,6 +7,11 @@ use sha2::{Digest, Sha256};
 pub struct RequestFingerprint([u8; 32]);
 
 impl RequestFingerprint {
+    #[must_use]
+    pub fn as_bytes(&self) -> &[u8; 32] {
+        &self.0
+    }
+
     #[must_use]
     pub fn to_hex(self) -> String {
         const HEX: &[u8; 16] = b"0123456789abcdef";