Apply coredata rules: idempotent deletes and constraint checks

Delete methods no longer check RowsAffected — deletes are
idempotent. PgError handlers now check both error code and
constraint name to avoid misattributing violations. Also
migrated remaining errors.As patterns to errors.AsType.

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

pkg/coredata/membership.go

@@ -138,8 +138,7 @@ VALUES (
 
 	result, err := conn.Exec(ctx, query, args)
 	if err != nil {
-		var pgErr *pgconn.PgError
-		if errors.As(err, &pgErr) && pgErr.Code == "23505" {
+		if pgErr, ok := errors.AsType[*pgconn.PgError](err); ok && pgErr.Code == "23505" && pgErr.ConstraintName == "authz_memberships_user_id_organization_id_key" {
 			return ErrResourceAlreadyExists
 		}
 
@@ -339,15 +338,11 @@ WHERE
 	}
 	maps.Copy(args, scope.SQLArguments())
 
-	result, err := conn.Exec(ctx, query, args)
+	_, err := conn.Exec(ctx, query, args)
 	if err != nil {
 		return fmt.Errorf("cannot delete membership: %w", err)
 	}
 
-	if result.RowsAffected() == 0 {
-		return ErrResourceNotFound
-	}
-
 	return nil
 }