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

features/auth/routes.go

@@ -1,8 +1,10 @@
+// Package auth handles user authentication routes and handlers.
 package auth
 
 import (
 	"github.com/alexedwards/scs/v2"
 	"github.com/go-chi/chi/v5"
+
 	"github.com/ryanhamamura/c4/db/repository"
 )