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)

game/store.go

@@ -49,7 +49,7 @@ func (gs *GameStore) Create() *GameInstance {
 	gs.gamesMu.Unlock()
 
 	if gs.queries != nil {
-		gs.saveGame(gi.game) //nolint:errcheck
+		saveGame(gs.queries, gi.game) //nolint:errcheck
 	}
 
 	return gi
@@ -68,12 +68,12 @@ func (gs *GameStore) Get(id string) (*GameInstance, bool) {
 		return nil, false
 	}
 
-	g, err := gs.loadGame(id)
+	g, err := loadGame(gs.queries, id)
 	if err != nil || g == nil {
 		return nil, false
 	}
 
-	players, _ := gs.loadGamePlayers(id)
+	players, _ := loadGamePlayers(gs.queries, id)
 	for _, p := range players {
 		switch p.Color {
 		case 1:
@@ -152,8 +152,8 @@ func (gi *GameInstance) Join(ps *PlayerSession) bool {
 	}
 
 	if gi.queries != nil {
-		gi.saveGamePlayer(gi.game.ID, ps.Player, slot) //nolint:errcheck
-		gi.saveGame(gi.game)                           //nolint:errcheck
+		saveGamePlayer(gi.queries, gi.game.ID, ps.Player, slot) //nolint:errcheck
+		saveGame(gi.queries, gi.game)                           //nolint:errcheck
 	}
 
 	gi.notify()
@@ -190,7 +190,7 @@ func (gi *GameInstance) CreateRematch(gs *GameStore) *GameInstance {
 	gi.game.RematchGameID = &newID
 
 	if gi.queries != nil {
-		if err := gi.saveGame(gi.game); err != nil {
+		if err := saveGame(gi.queries, gi.game); err != nil {
 			gs.Delete(newID) //nolint:errcheck
 			gi.game.RematchGameID = nil
 			return nil
@@ -224,7 +224,7 @@ func (gi *GameInstance) DropPiece(col int, playerColor int) bool {
 	}
 
 	if gi.queries != nil {
-		gi.saveGame(gi.game) //nolint:errcheck
+		saveGame(gi.queries, gi.game) //nolint:errcheck
 	}
 
 	gi.notify()