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()

features/lobby/handlers.go

@@ -2,10 +2,10 @@ package lobby
 
 import (
 	"context"
-	"database/sql"
 	"fmt"
 	"net/http"
 	"strconv"
+	"time"
 
 	"github.com/ryanhamamura/c4/db/repository"
 	lobbycomponents "github.com/ryanhamamura/c4/features/lobby/components"
@@ -28,16 +28,24 @@ func HandleLobbyPage(queries *repository.Queries, sessions *scs.SessionManager,
 		var userGames []lobbycomponents.GameListItem
 		if isLoggedIn {
 			ctx := context.Background()
-			games, err := queries.GetUserActiveGames(ctx, sql.NullString{String: userID, Valid: true})
+			games, err := queries.GetUserActiveGames(ctx, &userID)
 			if err == nil {
 				for _, g := range games {
 					isMyTurn := g.Status == 1 && g.CurrentTurn == g.MyColor
+					opponentName := ""
+					if g.OpponentNickname != nil {
+						opponentName = *g.OpponentNickname
+					}
+					var lastPlayed time.Time
+					if g.UpdatedAt != nil {
+						lastPlayed = *g.UpdatedAt
+					}
 					userGames = append(userGames, lobbycomponents.GameListItem{
 						ID:           g.ID,
 						Status:       int(g.Status),
-						OpponentName: g.OpponentNickname.String,
+						OpponentName: opponentName,
 						IsMyTurn:     isMyTurn,
-						LastPlayed:   g.UpdatedAt.Time,
+						LastPlayed:   lastPlayed,
 					})
 				}
 			}