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/logic.go

@@ -1,3 +1,4 @@
+// Package game implements Connect 4 game logic, state management, and persistence.
 package game
 
 // DropPiece attempts to drop a piece in the given column.