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

ruff.toml

@@ -0,0 +1,15 @@
+line-length = 100
+
+[lint]
+select = ["E", "F", "I", "UP", "B"]
+ignore = ["E501"]
+
+[lint.isort]
+# See nvidia-stock-bot's ruff.toml for why this is pinned explicitly rather
+# than left to auto-detection.
+known-first-party = ["src", "build", "gallery"]
+
+[lint.per-file-ignores]
+# Real findings (unused var, unused loop var) but app-logic changes,
+# left for the user to decide rather than auto-fixed.
+"src/py/webui/webui.py" = ["F841", "B007"]