chore: declare license-files in pyproject.toml

Associates LICENSE.md with the package for pip/PyPI.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pyproject.toml

@@ -9,6 +9,7 @@ description = "Heartbeat monitoring system — client (hbc) and server (hbd)"
 readme = "README.md"
 requires-python = ">=3.11"
 license = "MIT"
+license-files = ["LICENSE.md"]
 keywords = ["heartbeat", "monitoring", "dns", "websocket", "system-monitoring"]
 authors = [
   { name = "Andreas Wrede" }