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)
- pytest --cov-fail-under=75 gate on the test stage
- 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 +1,10 @@
+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 = ["blocklist_scheduler"]