Add secret scanning, Dockerfile lint, ruff lint/format gate, coverage gate, and automatic CVE remediation PRs

- gitleaks (via docker cp, dockerignore-agnostic) and hadolint scan every push/PR
- new ruff lint stage (ruff.toml pins known-first-party for host/container
  consistency; F841/B007 in webui.py left un-fixed via per-file-ignores —
  app-logic changes, flagged for review rather than auto-fixed)
- pytest --cov-fail-under=90 gate on the test stage (already had pytest-cov,
  never wired into CI until now)
- scheduled Trivy critical failures now attempt an apk upgrade rebuild and open a PR
  if it clears the finding, instead of just failing red
- ruff --fix/--format applied to existing code to start the gate clean

tests/test_builder_utils.py

@@ -1,5 +1,3 @@
-from pathlib import Path
-
 from src.py.builder import utils
 
 
@@ -30,7 +28,7 @@ def test_load_theme_config_missing_raises(tmp_path):
     themes_dir = tmp_path / "themes"
     try:
         utils.load_theme_config("missing", themes_dir)
-        assert False, "expected FileNotFoundError"
+        raise AssertionError("expected FileNotFoundError")
     except FileNotFoundError:
         pass