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/pgsql/templates.go

@@ -164,6 +164,7 @@ type CreateForeignKeyData struct {
 	TargetColumns  string
 	OnDelete       string
 	OnUpdate       string
+	Deferral       string // DEFERRABLE / DEFERRABLE INITIALLY DEFERRED / NOT DEFERRABLE, or empty
 }
 
 // DropConstraintData contains data for drop constraint template
@@ -238,6 +239,7 @@ type CreateUniqueConstraintData struct {
 	TableName      string
 	ConstraintName string
 	Columns        string
+	UsingIndex     string // when set, promotes this existing unique index instead of building a new one
 }
 
 // CreateCheckConstraintData contains data for create check constraint template
@@ -259,7 +261,7 @@ type CreateForeignKeyWithCheckData struct {
 	TargetColumns  string
 	OnDelete       string
 	OnUpdate       string
-	Deferrable     bool
+	Deferral       string // DEFERRABLE / DEFERRABLE INITIALLY DEFERRED / NOT DEFERRABLE, or empty
 }
 
 // SetSequenceValueData contains data for set sequence value template