Replace xmax upsert trick with RETURNING full row

Upsert methods now RETURNING all struct columns and scan the result
back into the pointer receiver, keeping the caller in sync with the
actual DB state (id, created_at, etc. from the existing row on
conflict). Insert detection compares the saved original ID with the
returned ID instead of relying on the PostgreSQL-internal xmax column.

Signed-off-by: Émile Ré <emile@probo.com>

pkg/coredata/common_third_party_domain.go

@@ -118,7 +118,7 @@ INSERT INTO common_third_party_domains (
 	return nil
 }
 
-func (d CommonThirdPartyDomain) Upsert(
+func (d *CommonThirdPartyDomain) Upsert(
 	ctx context.Context,
 	conn pg.Tx,
 ) (inserted bool, err error) {
@@ -139,9 +139,16 @@ INSERT INTO common_third_party_domains (
 ON CONFLICT (common_third_party_id, domain) DO UPDATE
 SET
     updated_at = EXCLUDED.updated_at
-RETURNING (xmax = 0) AS inserted
+RETURNING
+    id,
+    common_third_party_id,
+    domain,
+    created_at,
+    updated_at
 `
 
+	originalID := d.ID
+
 	args := pgx.StrictNamedArgs{
 		"id":                    d.ID,
 		"common_third_party_id": d.CommonThirdPartyID,
@@ -156,12 +163,14 @@ RETURNING (xmax = 0) AS inserted
 	}
 	defer rows.Close()
 
-	inserted, err = pgx.CollectExactlyOneRow(rows, pgx.RowTo[bool])
+	row, err := pgx.CollectExactlyOneRow(rows, pgx.RowToStructByName[CommonThirdPartyDomain])
 	if err != nil {
 		return false, fmt.Errorf("cannot collect upsert result: %w", err)
 	}
 
-	return inserted, nil
+	*d = row
+
+	return originalID == d.ID, nil
 }
 
 func (d CommonThirdPartyDomain) Delete(