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/writers/dbml/writer.go

@@ -273,6 +273,13 @@ func (w *Writer) constraintToDBML(c *models.Constraint, t *models.Table) string
 		actions = append(actions, fmt.Sprintf("update: %s", c.OnUpdate))
 	}
 
+	if c.Deferrable != nil && *c.Deferrable {
+		actions = append(actions, "deferrable")
+	}
+	if c.InitiallyDeferred != nil && *c.InitiallyDeferred {
+		actions = append(actions, "initially deferred")
+	}
+
 	refLine := fmt.Sprintf("Ref: %s %s %s", fromRef, relationship, toRef)
 	if len(actions) > 0 {
 		refLine += fmt.Sprintf(" [%s]", strings.Join(actions, ", "))