feat: integrate security audits into pre-commit and CI/CD with backend pip-audit migration (#74)

## Summary

This PR integrates security-focused checks into the existing quality gates and aligns local workflows with CI/CD execution.

## What changed

- Added Bandit to pre-commit backend checks.
- Integrated eslint-plugin-security into the existing frontend ESLint setup used by pre-commit.
- Added dedicated audit tasks:
  - backend audit via pip-audit
  - frontend audit via yarn npm audit
- Updated CI/CD workflow to include and gate on frontend/backend audit jobs.
- Switched backend vulnerability scanning from Safety to pip-audit to avoid interactive/auth requirements in CI.
- Updated backend dependency set and lockfile to resolve test dependency conflicts and keep the environment solvable.
- Marked backend integration API tests with the integration marker so marker-based unit/integration separation works consistently.

## Validation

- Pre-commit hooks run and pass after formatting/autofixes.
- Branch commit created successfully after hook-driven file updates.
- Branch pushed to remote and tracking is configured.

## Notes

- pip-audit now executes from backend context (for example via uv --directory backend run ...), matching project layout.
- Remaining reported vulnerabilities depend on upstream package fix availability/constraints and may require follow-up once publishable fix versions are consumable.

## Follow-ups (optional)

- Add a curated pip-audit ignore policy for non-actionable/transient advisories with rationale.
- Open a focused follow-up PR for remaining dependency advisories once upstream fixes are practically installable.

Co-authored-by: copilotcoder <copilotcoder@darkhelm.org>
Reviewed-on: #74

scripts/run-ci-e2e-compose.sh

@@ -34,5 +34,5 @@ docker run --rm \
   -e PLAYWRIGHT_JUNIT_OUTPUT_FILE=/tmp/playwright-artifacts/playwright-results.xml \
   -v "${ROOT_DIR}/frontend:/workspace/frontend" \
   -v "${E2E_ARTIFACT_DIR}:/tmp/playwright-artifacts" \
-  mcr.microsoft/playwright:v1.56.1-jammy \
+  mcr.microsoft.com/playwright:v1.56.1-jammy \
   bash -lc "cd /workspace/frontend && corepack enable && yarn install --immutable && yarn test:e2e --reporter=list --timeout=90000"