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

game/persist.go

@@ -126,7 +126,7 @@ func gameFromRow(row repository.Game) (*Game, error) {
 	}
 
 	if row.WinningCells.Valid {
-		g.WinningCellsFromJSON(row.WinningCells.String)
+		_ = g.WinningCellsFromJSON(row.WinningCells.String)
 	}
 
 	if row.RematchGameID.Valid {