Validate the reconnect secret

Right now the secret is ignored as we don't check if any actual rows
have been updated (with the `WHERE secret = ?`).

I tried writing a test for this but we don't have an easy way to change
the secret during a test.

This issue was found by OpenAI o3.

Author: erikdubbelboer

internal/signaling/stores/postgres.go

@@ -419,7 +419,7 @@ func (s *PostgresStore) MarkPeerAsDisconnected(ctx context.Context, peerID strin
 func (s *PostgresStore) MarkPeerAsReconnected(ctx context.Context, peerID, secret, gameID string) (bool, []string, error) {
 	now := util.NowUTC(ctx)
 
-	_, err := s.DB.Exec(ctx, `
+	result, err := s.DB.Exec(ctx, `
 		UPDATE peers
 		SET
 			disconnected = FALSE,
@@ -432,6 +432,9 @@ func (s *PostgresStore) MarkPeerAsReconnected(ctx context.Context, peerID, secre
 	if err != nil {
 		return false, nil, err
 	}
+	if result.RowsAffected() == 0 {
+		return false, nil, nil
+	}
 
 	var lobbies []string
 	rows, err := s.DB.Query(ctx, `