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

Dockerfile

@@ -27,6 +27,13 @@ RUN pip install --no-cache-dir -r requirements-dev.txt
 COPY ./tests/ ./tests/
 COPY ./demo/ ./demo/
 
+FROM base AS lint
+
+RUN pip install --no-cache-dir ruff==0.16.4
+COPY ruff.toml /app/ruff.toml
+COPY ./tests/ ./tests/
+RUN ruff check . && ruff format --check .
+
 FROM base
 
 ENTRYPOINT ["/app/entrypoint.sh"]