refactor: deduplicate persistence, add upsert queries, throttle snake saves

- Replace Create+Get+Update with UpsertGame/UpsertSnakeGame queries
- Extract free functions (saveGame, loadGame, etc.) from duplicated
  receiver methods on Store and Instance types
- Remove duplicate generateID from snake package, reuse game.GenerateID
- Throttle snake game DB writes to every 2s instead of every tick
- Fix double-lock in c4game chat handler
- Update all code for sqlc pointer types (*string instead of sql.NullString)

db/repository/chat.sql.go

@@ -15,11 +15,11 @@ VALUES (?, ?, ?, ?, ?)
 `
 
 type CreateChatMessageParams struct {
-	GameID    string
-	Nickname  string
-	Color     int64
-	Message   string
-	CreatedAt int64
+	GameID    string `db:"game_id" json:"game_id"`
+	Nickname  string `db:"nickname" json:"nickname"`
+	Color     int64  `db:"color" json:"color"`
+	Message   string `db:"message" json:"message"`
+	CreatedAt int64  `db:"created_at" json:"created_at"`
 }
 
 func (q *Queries) CreateChatMessage(ctx context.Context, arg CreateChatMessageParams) error {
@@ -40,13 +40,13 @@ ORDER BY created_at DESC, id DESC
 LIMIT 50
 `
 
-func (q *Queries) GetChatMessages(ctx context.Context, gameID string) ([]ChatMessage, error) {
+func (q *Queries) GetChatMessages(ctx context.Context, gameID string) ([]*ChatMessage, error) {
 	rows, err := q.db.QueryContext(ctx, getChatMessages, gameID)
 	if err != nil {
 		return nil, err
 	}
 	defer rows.Close()
-	var items []ChatMessage
+	var items []*ChatMessage
 	for rows.Next() {
 		var i ChatMessage
 		if err := rows.Scan(
@@ -59,7 +59,7 @@ func (q *Queries) GetChatMessages(ctx context.Context, gameID string) ([]ChatMes
 		); err != nil {
 			return nil, err
 		}
-		items = append(items, i)
+		items = append(items, &i)
 	}
 	if err := rows.Close(); err != nil {
 		return nil, err