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

router/router.go

@@ -40,10 +40,18 @@ func SetupRoutes(
 		setupReload(router)
 	}
 
-	auth.SetupRoutes(router, queries, sessions)
-	lobby.SetupRoutes(router, queries, sessions, store, snakeStore)
-	c4game.SetupRoutes(router, store, nc, sessions, queries)
-	snakegame.SetupRoutes(router, snakeStore, nc, sessions)
+	if err := auth.SetupRoutes(router, queries, sessions); err != nil {
+		return err
+	}
+	if err := lobby.SetupRoutes(router, queries, sessions, store, snakeStore); err != nil {
+		return err
+	}
+	if err := c4game.SetupRoutes(router, store, nc, sessions, queries); err != nil {
+		return err
+	}
+	if err := snakegame.SetupRoutes(router, snakeStore, nc, sessions); err != nil {
+		return err
+	}
 
 	return nil
 }