fix: add coverage configuration and test infrastructure for push and utils modules

Add coverage tooling to CI workflow, Makefile, and pyproject.toml dependencies.
Introduce new test suites for push notification helpers (base64 encoding, HKDF,
authorization JWT) and utility functions (safe_next, strip_html, mention extraction,
badge/xp awarding, milestone checks) with a session-scoped local_db fixture.

pyproject.toml

@@ -25,7 +25,7 @@ dependencies = [
 devplace = "devplacepy.cli:main"
 
 [project.optional-dependencies]
-dev = ["pytest", "playwright", "pytest-xdist", "requests"]
+dev = ["pytest", "playwright", "pytest-xdist", "requests", "coverage"]
 
 [build-system]
 requires = ["hatchling"]