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, ", "))

pkg/writers/dbml/writer_test.go

@@ -171,3 +171,31 @@ func TestWriter_WriteDatabase_OneToOneRelationship(t *testing.T) {
 
 	assert.Contains(t, output, "Ref: public.profiles.user_id - public.users.id")
 }
+
+func TestWriter_WriteDatabase_ForeignKeyDeferral(t *testing.T) {
+	yes := true
+	db := models.InitDatabase("test_db")
+	schema := models.InitSchema("public")
+	users := models.InitTable("users", "public")
+	users.Columns["id"] = models.InitColumn("id", "users", "public")
+	users.Columns["id"].Type = "bigint"
+	posts := models.InitTable("posts", "public")
+	posts.Columns["user_id"] = models.InitColumn("user_id", "posts", "public")
+	posts.Columns["user_id"].Type = "bigint"
+	fk := models.InitConstraint("fk_posts_user", models.ForeignKeyConstraint)
+	fk.Table, fk.Schema = "posts", "public"
+	fk.Columns = []string{"user_id"}
+	fk.ReferencedTable, fk.ReferencedSchema = "users", "public"
+	fk.ReferencedColumns = []string{"id"}
+	fk.Deferrable = &yes
+	fk.InitiallyDeferred = &yes
+	posts.Constraints[fk.Name] = fk
+	schema.Tables = append(schema.Tables, users, posts)
+	db.Schemas = append(db.Schemas, schema)
+
+	outputPath := filepath.Join(t.TempDir(), "test.dbml")
+	assert.NoError(t, NewWriter(&writers.WriterOptions{OutputPath: outputPath}).WriteDatabase(db))
+	content, err := os.ReadFile(outputPath)
+	assert.NoError(t, err)
+	assert.Contains(t, string(content), "Ref: public.posts.user_id > public.users.id [deferrable, initially deferred]")
+}