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.go

@@ -512,7 +512,7 @@ func formatColumnDefaultSQL(col *models.Column) (setDefault bool, defaultVal str
 		return false, ""
 	}
 	if value, ok := col.Default.(string); ok {
-		return true, writers.QuoteDefaultValue(value, col.Type)
+		return true, writers.QuoteDefaultValue(stripBackticks(value), col.Type)
 	}
 	return true, fmt.Sprintf("%v", col.Default)
 }