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

Dockerfile

@@ -15,9 +15,15 @@ COPY blocklist_scheduler.py entrypoint.sh VERSION ./
 RUN chmod +x entrypoint.sh
 
 FROM base AS test
-RUN pip install --no-cache-dir pytest==9.1.1
+RUN pip install --no-cache-dir pytest==9.1.1 pytest-cov==7.1.0
 COPY tests/ tests/
 COPY pytest.ini .
 
+FROM base AS lint
+RUN pip install --no-cache-dir ruff==0.16.4
+COPY ruff.toml .
+COPY tests/ tests/
+RUN ruff check . && ruff format --check .
+
 FROM base
 ENTRYPOINT ["./entrypoint.sh"]