fix(merge,pgsql): treat serial types as their base integer in diffs, unquote bare keyword defaults

Merge conflict detection compared bigserial (DBML) against bigint (live
PostgreSQL read of an existing serial column) as incompatible types, since
serial is sugar over an integer column plus a sequence default and
PostgreSQL always reports back the underlying integer type. Add
SerialUnderlyingType and use it when comparing column types for conflicts.

QuoteDefaultValue also wrapped bare keyword expressions like CURRENT_DATE
in string quotes because they contain no parentheses, unlike function-call
defaults such as now(). Recognize known bare keyword defaults and leave
them unquoted across CREATE TABLE, ALTER TABLE ADD COLUMN, and
ALTER COLUMN SET DEFAULT generation.

pkg/pgsql/datatypes.go

@@ -193,6 +193,28 @@ func IsKnownPGBaseType(baseType string) bool {
 	return ok
 }
 
+// serialUnderlyingType maps each serial pseudo-type to the integer type
+// PostgreSQL actually stores the column as. serial/bigserial/smallserial are
+// not real types: they are sugar for an integer column plus a sequence
+// default, and pg_catalog (and information_schema) always reports the
+// underlying integer type back for such columns.
+var serialUnderlyingType = map[string]string{
+	"serial":      "integer",
+	"bigserial":   "bigint",
+	"smallserial": "smallint",
+}
+
+// SerialUnderlyingType returns the underlying integer type for a serial
+// pseudo-type (e.g. "bigserial" -> "bigint"). If baseType (after
+// NormalizePGType) is not a serial type, it is returned unchanged.
+func SerialUnderlyingType(baseType string) string {
+	normalized := NormalizePGType(baseType)
+	if underlying, ok := serialUnderlyingType[normalized]; ok {
+		return underlying
+	}
+	return normalized
+}
+
 func IsGoType(pTypeName string) bool {
 	for k := range GoToStdTypes {
 		if strings.EqualFold(pTypeName, k) {