Replace zerolog with log/slog from stdlib

- Rewrite logger package to use slog with LevelVar for dynamic levels
- Update all packages to use *slog.Logger instead of *zerolog.Logger
- Use TextHandler for TTY (dev), JSONHandler for production
- Add make check target (runs lint + test)
- Add make test target

Makefile

@@ -24,6 +24,11 @@ lint:
 	golangci-lint run
 	sh -c 'test -z "$$(gofmt -l .)"'
 
+test:
+	go test ./...
+
+check: lint test
+
 debug: ./$(FN)d
 	DEBUG=1 GOTRACEBACK=all ./$(FN)d