Fix `created_at` default val in v4 migration for SQLite

This previously used a default timestamp value which caused the
migration to fail for SQLite databases.

db/create.go

@@ -139,6 +139,15 @@ func (c *Column) SetDefault(value string) *Column {
 	return c
 }
 
+func (c *Column) SetDefaultCurrentTimestamp() *Column {
+	def := "NOW()"
+	if c.Dialect == DialectSQLite {
+		def = "CURRENT_TIMESTAMP"
+	}
+	c.Default = OptionalString{Set: true, Value: def}
+	return c
+}
+
 func (c *Column) SetType(t ColumnType) *Column {
 	c.Type = t
 	return c

migrations/v4.go

@@ -29,7 +29,7 @@ func oauth(db *datastore) error {
 			SetIfNotExists(true).
 			Column(dialect.Column("state", wf_db.ColumnTypeVarChar, wf_db.OptionalInt{Set: true, Value: 255})).
 			Column(dialect.Column("used", wf_db.ColumnTypeBool, wf_db.UnsetSize)).
-			Column(dialect.Column("created_at", wf_db.ColumnTypeDateTime, wf_db.UnsetSize).SetDefault("NOW()")).
+			Column(dialect.Column("created_at", wf_db.ColumnTypeDateTime, wf_db.UnsetSize).SetDefaultCurrentTimestamp()).
 			UniqueConstraint("state").
 			ToSQL()
 		if err != nil {