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/merge/merge.go

@@ -605,8 +605,8 @@ func cloneConstraint(constraint *models.Constraint) *models.Constraint {
 		OnDelete:          constraint.OnDelete,
 		Expression:        constraint.Expression,
 		Name:              constraint.Name,
-		Deferrable:        constraint.Deferrable,
-		InitiallyDeferred: constraint.InitiallyDeferred,
+		Deferrable:        copyBoolPtr(constraint.Deferrable),
+		InitiallyDeferred: copyBoolPtr(constraint.InitiallyDeferred),
 		Sequence:          constraint.Sequence,
 	}
 	copy(newConstraint.Columns, constraint.Columns)
@@ -823,3 +823,11 @@ func min(a, b int) int {
 	}
 	return b
 }
+
+func copyBoolPtr(b *bool) *bool {
+	if b == nil {
+		return nil
+	}
+	v := *b
+	return &v
+}