perf: cache the format of feeds

Detecting the format of a feed accounts for up to 30% of the time spent in
`parser.ParseFeed`. Cache the value once detected, as it'll never change.

internal/database/migrations.go

@@ -1015,4 +1015,12 @@ var migrations = []func(tx *sql.Tx, driver string) error{
 		_, err = tx.Exec(sql)
 		return err
 	},
+	func(tx *sql.Tx, _ string) (err error) {
+		sql := `
+		ALTER TABLE feeds ADD COLUMN format text default '';
+		ALTER TABLE feeds ADD COLUMN format_version text default '';
+		`
+		_, err = tx.Exec(sql)
+		return err
+	},
 }