chore: scaffold Gitea Actions pipelines per ADR-0015 / ADR-0017

Wire the CI/CD pipeline scaffolding. Implements the level-2 (Gitea
Actions) part of ADR-0015 with the thin-YAML pattern, plus the perf
gate from ADR-0017. The level-1 decisions (gates list, branch model,
secrets policy) are unchanged.

Files:
- .nvmrc pins Node 24 (latest LTS major) for actions/setup-node.
- package.json gains four ci:* scripts that the workflows call:
  - ci:check  - 'nx affected -t format:check lint test build'
  - ci:audit  - 'pnpm audit --audit-level=moderate'
  - ci:commits- 'commitlint --from $COMMIT_LINT_FROM --to HEAD'
  - ci:perf   - 'nx build portal-shell --configuration=production
                 && lhci autorun --config=./lighthouserc.js'
  All four runnable locally; CI workflows are thin wrappers.
- @lhci/cli added as a dev dependency for ci:perf.
- lighthouserc.js encodes the Core Web Vitals thresholds from
  ADR-0017 (LCP <= 2500ms, CLS <= 0.1, TBT <= 200ms, server
  response <= 800ms, Performance >= 0.9). v1 measures only the
  static-served portal-shell bundle (Nx Welcome placeholder); the
  critical-routes list expands as real screens land.
- .gitea/workflows/ci.yml runs five jobs on PR + push to main:
  check, scan (audit + Trivy + gitleaks), commits (PR-only), perf,
  a11y. The a11y job is a placeholder that no-ops with a clear
  message; it wires up for real with the first Playwright e2e suite
  (ADR-0016). All gates are blocking - branch protection on main
  will require all five jobs green.
- .gitea/workflows/security-scheduled.yml runs weekly (Mon 04:00
  UTC) for full-tree Trivy + gitleaks (no severity filter, no
  skip-dirs - broader than per-PR) plus a Lighthouse run against
  the prod URL when vars.LHCI_PROD_URL is set.

Slight deviation from ADR-0015 §'Level 2': Trivy and gitleaks are
binaries (Go) and don't have clean npm wrappers, so they are
invoked through their official Gitea-Actions-compatible actions
inside the YAML rather than via 'pnpm ci:scan'. The high-level
decision (gates: audit + secret-scan + dep-scan) is unchanged; the
script/action boundary is shifted by one tool. Documented here for
traceability; no ADR amendment needed.

Operational TODOs (not blocking the scaffold):
- pnpm audit currently reports 6 moderate transitive vulnerabilities
  (ajv, brace-expansion, yaml, @hono/node-server, follow-redirects,
  uuid) in deep deps of Nx/Angular plugins. CI will fail on this
  gate until upstream updates land or Renovate bumps; expected and
  documented.
- act_runner self-hosted instances are not yet registered against
  the Gitea organisation; the workflows reference [self-hosted,
  on-prem] runner labels per ADR-0015. CI will not actually execute
  until the runners are up - that's an infra task.
- Branch protection rules on main (require all five jobs green) are
  configured in Gitea UI, not in this commit.
- Lighthouse-prod scheduled job runs only when vars.LHCI_PROD_URL
  is set - skipped silently otherwise. To be configured once a prod
  environment exists.

.nvmrc

@@ -0,0 +1 @@
+24