fix(db): map 23505 unique_violation to DomainError::Conflict (→ HTTP 409); close TOCTOU on register save

crates/adapters/postgres/src/db_error.rs

@@ -6,6 +6,15 @@ pub(crate) trait IntoDbResult<T> {
 
 impl<T> IntoDbResult<T> for Result<T, sqlx::Error> {
     fn into_domain(self) -> Result<T, DomainError> {
-        self.map_err(|e| DomainError::Internal(e.to_string()))
+        self.map_err(|e| {
+            if let sqlx::Error::Database(ref db) = e {
+                if db.code().as_deref() == Some("23505") {
+                    return DomainError::Conflict(
+                        db.constraint().unwrap_or("conflict").to_string(),
+                    );
+                }
+            }
+            DomainError::Internal(e.to_string())
+        })
     }
 }