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; B905 in env_config.py's zip() left un-fixed — app-logic change,
  see feedback-no-app-logic-changes)
- 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 +1,13 @@
+line-length = 100
+
+[lint]
+select = ["E", "F", "I", "UP", "B"]
+# E501: handled by the formatter. B905: zip() without strict= in env_config.py —
+# app-logic change, left for the user to decide (see feedback-no-app-logic-changes).
+ignore = ["E501", "B905"]
+
+[lint.isort]
+# Pinned explicitly: auto-detection of first-party modules differs between the
+# host (app/ subdir + .git present) and the Docker lint stage (flattened to /app,
+# no .git) — without this, import-sort results silently diverge between the two.
+known-first-party = ["env_config", "gpu_checker", "notifier", "main"]