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/risk_assessment_scenario.go

@@ -351,7 +351,7 @@ INSERT INTO risk_assessment_scenarios (
 	}
 	_, err := conn.Exec(ctx, q, args)
 	if err != nil {
-		if pgErr, ok := errors.AsType[*pgconn.PgError](err); ok && pgErr.Code == "23505" {
+		if pgErr, ok := errors.AsType[*pgconn.PgError](err); ok && pgErr.Code == "23505" && pgErr.ConstraintName == "risk_assessment_scenarios_pkey" {
 			return ErrResourceAlreadyExists
 		}
 		return fmt.Errorf("cannot insert risk scenario: %w", err)