feat(pgsql): unique constraints on apply, matching unique index names, FK deferral

- migration writer now emits unique constraints (direct apply and live diff
  previously dropped them); idempotent via the existing DO block
- unique constraint adopts a same-named model unique index with
  ADD CONSTRAINT ... UNIQUE USING INDEX instead of building a second index
- dbml reader names unnamed unique indexes ukey_<table>_<cols> to match the
  unique constraint naming
- foreign key deferral: Constraint.Deferrable/InitiallyDeferred are *bool
  (nil = unspecified, never diffed); writers emit DEFERRABLE, DEFERRABLE
  INITIALLY DEFERRED and NOT DEFERRABLE; live reader reads condeferrable and
  condeferred; dbml reader/writer support deferrable and initially deferred
- tests and README/naming docs updated

pkg/diff/diff.go

@@ -504,6 +504,9 @@ func compareConstraintDetails(source, target *models.Constraint) map[string]any
 	if normalizeConstraintAction(source.OnUpdate) != normalizeConstraintAction(target.OnUpdate) {
 		changes["on_update"] = map[string]string{"source": source.OnUpdate, "target": target.OnUpdate}
 	}
+	if !models.DeferralMatches(source, target) {
+		changes["deferral"] = map[string]string{"source": source.DeferralClause(), "target": target.DeferralClause()}
+	}
 
 	return changes
 }