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/gen/models.go

@@ -9,14 +9,15 @@ import (
 )
 
 type Game struct {
-	ID           string
-	Board        string
-	CurrentTurn  int64
-	Status       int64
-	WinnerUserID sql.NullString
-	WinningCells sql.NullString
-	CreatedAt    sql.NullTime
-	UpdatedAt    sql.NullTime
+	ID            string
+	Board         string
+	CurrentTurn   int64
+	Status        int64
+	WinnerUserID  sql.NullString
+	WinningCells  sql.NullString
+	CreatedAt     sql.NullTime
+	UpdatedAt     sql.NullTime
+	RematchGameID sql.NullString
 }
 
 type GamePlayer struct {