Refine PgError constraint checks and document PK rule

Remove dead 23505 checks on single-GID primary keys (oauth2_consent,
risk_assessment, risk_assessment_scenario, risk_assessment_scope).
Add missing constraints to membership_profile and
statement_of_applicability. Document composite-PK vs GID-PK rule in
cursor rules and contrib guide.

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

pkg/coredata/oauth2_consent.go

@@ -22,7 +22,6 @@ import (
 	"time"
 
 	"github.com/jackc/pgx/v5"
-	"github.com/jackc/pgx/v5/pgconn"
 	"go.gearno.de/kit/pg"
 	"go.probo.inc/probo/pkg/gid"
 	"go.probo.inc/probo/pkg/page"
@@ -367,10 +366,6 @@ INSERT INTO iam_oauth2_consents (
 
 	_, err := conn.Exec(ctx, q, args)
 	if err != nil {
-		if pgErr, ok := errors.AsType[*pgconn.PgError](err); ok && pgErr.Code == "23505" && pgErr.ConstraintName == "iam_oauth2_consents_pkey" {
-			return ErrResourceAlreadyExists
-		}
-
 		return fmt.Errorf("cannot insert oauth2_consent: %w", err)
 	}