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/migrations/002_add_rematch.sql

@@ -0,0 +1,5 @@
+-- +goose Up
+ALTER TABLE games ADD COLUMN rematch_game_id TEXT;
+
+-- +goose Down
+ALTER TABLE games DROP COLUMN rematch_game_id;