feat(account): journal detached credentials to the retention log

On unlink (RemoveIdentity, reason=unlink) and email change (replaceEmailIdentity,
reason=change) write the outgoing credential to retained_identities before removing
the live identities row — so the legal dossier survives while the (kind, external_id)
frees for reuse. Same transaction, so the dossier and live state cannot diverge.
Integration tests cover both reasons.

backend/internal/account/email.go

@@ -546,6 +546,25 @@ func (s *Store) replaceEmailIdentity(ctx context.Context, confirmationID, accoun
 		if _, err := upd.ExecContext(ctx, tx); err != nil {
 			return fmt.Errorf("consume confirmation: %w", err)
 		}
+		// Journal the outgoing email before replacing it, so the legal dossier keeps the
+		// address the account used to hold (see retention.go).
+		var old model.Identities
+		sel := postgres.SELECT(
+			table.Identities.ExternalID, table.Identities.Confirmed, table.Identities.CreatedAt,
+		).FROM(table.Identities).WHERE(
+			table.Identities.AccountID.EQ(postgres.UUID(accountID)).
+				AND(table.Identities.Kind.EQ(postgres.String(KindEmail))),
+		).LIMIT(1)
+		switch err := sel.QueryContext(ctx, tx, &old); {
+		case err == nil:
+			if err := retainIdentityTx(ctx, tx, accountID, KindEmail, old.ExternalID, old.Confirmed, old.CreatedAt, retainChange); err != nil {
+				return err
+			}
+		case errors.Is(err, qrm.ErrNoRows):
+			// No prior email (this doubles as an attach); nothing to retain.
+		default:
+			return fmt.Errorf("load outgoing email identity: %w", err)
+		}
 		del := table.Identities.DELETE().WHERE(
 			table.Identities.AccountID.EQ(postgres.UUID(accountID)).
 				AND(table.Identities.Kind.EQ(postgres.String(KindEmail))),