feat(models): add support for generated and identity columns

* extend Column struct with Generated, GenerationExpression, Identity, and IdentityGeneration fields
* update queryColumns to handle new column properties
* modify migration writer to avoid altering generated and identity columns
* add tests to ensure correct handling of generated and identity columns

pkg/writers/pgsql/writer_test.go

@@ -1510,3 +1510,42 @@ func TestIndexStorageParameters(t *testing.T) {
 		})
 	}
 }
+
+func TestGenerateColumnDefinition_GeneratedColumnEmitsGeneratedClauseNotDefault(t *testing.T) {
+	w := &Writer{}
+
+	col := models.InitColumn("geom", "info_city", "public")
+	col.Type = "text"
+	col.NotNull = true
+	col.Generated = true
+	col.GenerationExpression = "st_makepoint(lon, lat)"
+	// A generated column's introspected "default" is actually its generation
+	// expression surfaced via pg_attrdef; it must never be rendered as DEFAULT.
+	col.Default = "st_makepoint(lon, lat)"
+
+	got := w.generateColumnDefinition(col)
+
+	want := "geom text GENERATED ALWAYS AS (st_makepoint(lon, lat)) STORED NOT NULL"
+	if got != want {
+		t.Fatalf("generateColumnDefinition() = %q, want %q", got, want)
+	}
+}
+
+func TestGenerateColumnDefinition_IdentityColumnEmitsIdentityClauseNotDefault(t *testing.T) {
+	w := &Writer{}
+
+	col := models.InitColumn("id", "users", "public")
+	col.Type = "bigint"
+	col.NotNull = true
+	col.Identity = true
+	col.IdentityGeneration = "ALWAYS"
+	// Identity columns never carry a literal default; this must never surface.
+	col.Default = "some-stray-value"
+
+	got := w.generateColumnDefinition(col)
+
+	want := "id bigint GENERATED ALWAYS AS IDENTITY NOT NULL"
+	if got != want {
+		t.Fatalf("generateColumnDefinition() = %q, want %q", got, want)
+	}
+}