Add play again button for rematch after game ends

When a game finishes (win or draw), players see a "Play again" button.
Clicking it creates a new game and the opponent sees a "Join Rematch"
link to join the same game.

db/queries/games.sql

@@ -8,7 +8,7 @@ SELECT * FROM games WHERE id = ?;
 
 -- name: UpdateGame :exec
 UPDATE games
-SET board = ?, current_turn = ?, status = ?, winner_user_id = ?, winning_cells = ?, updated_at = CURRENT_TIMESTAMP
+SET board = ?, current_turn = ?, status = ?, winner_user_id = ?, winning_cells = ?, rematch_game_id = ?, updated_at = CURRENT_TIMESTAMP
 WHERE id = ?;
 
 -- name: DeleteGame :exec