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/models/models.go

@@ -329,8 +329,8 @@ type Constraint struct {
 	ReferencedColumns []string       `json:"referenced_columns" yaml:"referenced_columns" xml:"referenced_columns"`
 	OnDelete          string         `json:"on_delete" yaml:"on_delete" xml:"on_delete"` // CASCADE, SET NULL, RESTRICT, etc.
 	OnUpdate          string         `json:"on_update" yaml:"on_update" xml:"on_update"`
-	Deferrable        bool           `json:"deferrable,omitempty" yaml:"deferrable,omitempty" xml:"deferrable,omitempty"`
-	InitiallyDeferred bool           `json:"initially_deferred,omitempty" yaml:"initially_deferred,omitempty" xml:"initially_deferred,omitempty"`
+	Deferrable        *bool          `json:"deferrable,omitempty" yaml:"deferrable,omitempty" xml:"deferrable,omitempty"`                         // nil: unspecified, left as is
+	InitiallyDeferred *bool          `json:"initially_deferred,omitempty" yaml:"initially_deferred,omitempty" xml:"initially_deferred,omitempty"` // nil: unspecified, left as is
 	Sequence          uint           `json:"sequence,omitempty" yaml:"sequence,omitempty" xml:"sequence,omitempty"`
 	GUID              string         `json:"guid" yaml:"guid" xml:"guid"`
 }