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)

features/c4game/handlers.go

@@ -156,9 +156,6 @@ func HandleGameEvents(store *game.GameStore, nc *nats.Conn, sessions *scs.Sessio
 				if len(chatMessages) > 50 {
 					chatMessages = chatMessages[len(chatMessages)-50:]
 				}
-				chatMu.Unlock()
-
-				chatMu.Lock()
 				msgs := make([]components.ChatMessage, len(chatMessages))
 				copy(msgs, chatMessages)
 				chatMu.Unlock()