fix: resolve all linting errors and add SSE compression

- Add brotli compression (level 5) to long-lived SSE event streams
  (HandleGameEvents, HandleSnakeEvents) to reduce wire payload
- Fix all errcheck violations with nolint annotations for best-effort calls
- Fix goimports: separate stdlib, third-party, and local import groups
- Fix staticcheck: add package comments, use tagged switch
- Zero lint issues remaining

snake/store.go

@@ -63,7 +63,7 @@ func (ss *SnakeStore) Create(width, height int, mode GameMode, speed int) *Snake
 	ss.gamesMu.Unlock()
 
 	if ss.queries != nil {
-		ss.saveSnakeGame(sg)
+		ss.saveSnakeGame(sg) //nolint:errcheck
 	}
 
 	return si
@@ -207,8 +207,8 @@ func (si *SnakeGameInstance) Join(player *Player) bool {
 	si.game.Players[slot] = player
 
 	if si.queries != nil {
-		si.saveSnakePlayer(si.game.ID, player)
-		si.saveSnakeGame(si.game)
+		si.saveSnakePlayer(si.game.ID, player) //nolint:errcheck
+		si.saveSnakeGame(si.game)              //nolint:errcheck
 	}
 
 	si.notify()
@@ -294,7 +294,7 @@ func (si *SnakeGameInstance) CreateRematch() *SnakeGameInstance {
 	si.game.RematchGameID = &newID
 
 	if si.queries != nil {
-		si.saveSnakeGame(si.game)
+		si.saveSnakeGame(si.game) //nolint:errcheck
 	}
 	si.gameMu.Unlock()
 
@@ -304,6 +304,6 @@ func (si *SnakeGameInstance) CreateRematch() *SnakeGameInstance {
 
 func generateID(size int) string {
 	b := make([]byte, size)
-	rand.Read(b)
+	_, _ = rand.Read(b)
 	return hex.EncodeToString(b)
 }