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/writers/writer.go

@@ -148,6 +148,26 @@ func SanitizeFilename(name string) string {
 // Examples (boolean):   "true"         → "true"
 // Examples (bigint):    "0"            → "0"
 // Examples (timestamp): "now()"        → "now()"   (function call – never quoted)
+// bareKeywordDefaults are PostgreSQL default-value keywords that are
+// expressions, not string literals, even though they contain no
+// parentheses (e.g. "CURRENT_DATE" rather than "now()"). They must never be
+// wrapped in quotes.
+var bareKeywordDefaults = map[string]bool{
+	"current_date":      true,
+	"current_time":      true,
+	"current_timestamp": true,
+	"localtime":         true,
+	"localtimestamp":    true,
+	"current_user":      true,
+	"session_user":      true,
+	"current_role":      true,
+	"current_catalog":   true,
+	"current_schema":    true,
+	"null":              true,
+	"true":              true,
+	"false":             true,
+}
+
 func QuoteDefaultValue(value, sqlType string) string {
 	value = strings.TrimSpace(value)
 
@@ -158,6 +178,12 @@ func QuoteDefaultValue(value, sqlType string) string {
 		return value
 	}
 
+	// Bare keyword expressions (e.g. CURRENT_DATE) are never quoted,
+	// regardless of column type.
+	if bareKeywordDefaults[strings.ToLower(value)] {
+		return value
+	}
+
 	// Normalise the SQL type: lowercase, strip length/precision suffix.
 	baseType := strings.ToLower(strings.TrimSpace(sqlType))
 	if idx := strings.Index(baseType, "("); idx > 0 {