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/types.go

@@ -100,7 +100,7 @@ type SnakeGame struct {
 	Speed         int      // cells per second
 }
 
-// Speed presets
+// SpeedPreset defines a named speed option for the snake game.
 type SpeedPreset struct {
 	Name  string
 	Speed int
@@ -129,7 +129,7 @@ func (sg *SnakeGame) PlayerCount() int {
 	return count
 }
 
-// Grid presets
+// GridPreset defines a named grid size option for the snake game.
 type GridPreset struct {
 	Name   string
 	Width  int
@@ -163,7 +163,7 @@ func (sg *SnakeGame) snapshot() *SnakeGame {
 	return &cp
 }
 
-// Snake colors (hex values for CSS)
+// SnakeColors are hex color values for CSS, indexed by player slot.
 var SnakeColors = []string{
 	"#00b894", // 1: Green
 	"#e17055", // 2: Orange