slim down image: python:3.10-slim base + python-based healthcheck

Base python:3.10 (917MB) dominated the 986MB image and kaniko's snapshot/push time. All deps are pure-python/wheels, so slim works without build tools. Replace the curl healthcheck (curl absent in slim) with a stdlib urllib check.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -1,4 +1,4 @@
-FROM python:3.10
+FROM python:3.10-slim
 
 #
 WORKDIR /code