core: type workspace and secret timestamps

crates/crank-registry/src/model.rs

@@ -61,7 +61,7 @@ pub struct MembershipRecord {
     pub workspace_id: WorkspaceId,
     pub user: User,
     pub role: MembershipRole,
-    pub created_at: String,
+    pub created_at: OffsetDateTime,
 }
 
 #[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
@@ -448,8 +448,8 @@ pub struct RotateSecretRequest<'a> {
     pub secret_id: &'a SecretId,
     pub ciphertext: &'a str,
     pub key_version: &'a str,
-    pub created_at: &'a str,
-    pub updated_at: &'a str,
+    pub created_at: &'a OffsetDateTime,
+    pub updated_at: &'a OffsetDateTime,
     pub created_by: Option<&'a crank_core::UserId>,
 }