fix(pgsql): strip backticks from column defaults in migration paths

Backtick-wrapped defaults (e.g. from GORM tags like `now()`) were only
stripped in the CREATE TABLE column-definition path, leaving raw
backticks in the ALTER COLUMN ... SET DEFAULT migration statement and
in the migration-generated CREATE TABLE template, producing invalid
SQL. Default-drift comparisons also compared raw values, so a
backtick-wrapped model default never matched the live DB default and
kept re-emitting redundant ALTER statements.

pkg/writers/pgsql/writer_test.go

@@ -1140,6 +1140,37 @@ func TestWriteSchema_EmitsGuardedAlterColumnDefaultStatements(t *testing.T) {
 	}
 }
 
+func TestWriteSchema_AlterColumnDefaultStripsBackticksFromFunctionExpression(t *testing.T) {
+	db := models.InitDatabase("testdb")
+	schema := models.InitSchema("public")
+
+	table := models.InitTable("agent_skills", "public")
+
+	updatedAtCol := models.InitColumn("updatedat", "agent_skills", "public")
+	updatedAtCol.Type = "timestamp"
+	updatedAtCol.Default = "`now()`"
+	table.Columns["updatedat"] = updatedAtCol
+
+	schema.Tables = append(schema.Tables, table)
+	db.Schemas = append(db.Schemas, schema)
+
+	var buf bytes.Buffer
+	writer := NewWriter(&writers.WriterOptions{})
+	writer.writer = &buf
+
+	if err := writer.WriteDatabase(db); err != nil {
+		t.Fatalf("WriteDatabase failed: %v", err)
+	}
+
+	output := buf.String()
+	if strings.Contains(output, "`") {
+		t.Fatalf("expected no backticks in generated SQL, got:\n%s", output)
+	}
+	if !strings.Contains(output, "ALTER COLUMN updatedat SET DEFAULT now()") {
+		t.Fatalf("expected guarded SET DEFAULT now() without backticks, got:\n%s", output)
+	}
+}
+
 func TestWriteSchema_GuardedAlterColumnTypeFallsBackOnConversionFailure(t *testing.T) {
 	db := models.InitDatabase("testdb")
 	schema := models.InitSchema("public")