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.

.gitea/workflows/ci.yml

@@ -0,0 +1,103 @@
+# Per ADR-0015 (CI/CD on Gitea Actions).
+# Thin YAML — orchestration lives in package.json scripts (ci:check,
+# ci:audit, ci:commits, ci:perf) and Nx targets. Any change to gate
+# behaviour belongs in those scripts, not in this file.
+
+name: CI
+
+on:
+  pull_request:
+    branches: [main]
+  push:
+    branches: [main]
+
+jobs:
+  check:
+    runs-on: [self-hosted, on-prem]
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+      - uses: nrwl/nx-set-shas@v4
+      - uses: pnpm/action-setup@v3
+      - uses: actions/setup-node@v4
+        with:
+          node-version-file: '.nvmrc'
+          cache: 'pnpm'
+      - run: pnpm install --frozen-lockfile
+      - run: pnpm ci:check
+
+  scan:
+    runs-on: [self-hosted, on-prem]
+    steps:
+      - uses: actions/checkout@v4
+      - uses: pnpm/action-setup@v3
+      - uses: actions/setup-node@v4
+        with:
+          node-version-file: '.nvmrc'
+          cache: 'pnpm'
+      - run: pnpm install --frozen-lockfile
+      - run: pnpm ci:audit
+      # Dependency vulnerability scan (binary tool, invoked via official
+      # action — Trivy is a Go binary, not an npm package, so it cannot
+      # live in package.json scripts as cleanly as audit/lint do).
+      - uses: aquasecurity/trivy-action@master
+        with:
+          scan-type: fs
+          ignore-unfixed: true
+          skip-dirs: node_modules
+          exit-code: '1'
+          severity: 'CRITICAL,HIGH'
+      # Secret scan, same reasoning (gitleaks is a Go binary).
+      - uses: gitleaks/gitleaks-action@v2
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+
+  commits:
+    if: github.event_name == 'pull_request'
+    runs-on: [self-hosted, on-prem]
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+      - uses: pnpm/action-setup@v3
+      - uses: actions/setup-node@v4
+        with:
+          node-version-file: '.nvmrc'
+          cache: 'pnpm'
+      - run: pnpm install --frozen-lockfile
+      - run: COMMIT_LINT_FROM=origin/main pnpm ci:commits
+
+  perf:
+    runs-on: [self-hosted, on-prem]
+    steps:
+      - uses: actions/checkout@v4
+      - uses: pnpm/action-setup@v3
+      - uses: actions/setup-node@v4
+        with:
+          node-version-file: '.nvmrc'
+          cache: 'pnpm'
+      - run: pnpm install --frozen-lockfile
+      - run: pnpm ci:perf
+      - uses: actions/upload-artifact@v4
+        if: always()
+        with:
+          name: lighthouseci-report
+          path: .lighthouseci/
+          retention-days: 30
+
+  a11y:
+    runs-on: [self-hosted, on-prem]
+    steps:
+      - uses: actions/checkout@v4
+      - uses: pnpm/action-setup@v3
+      - uses: actions/setup-node@v4
+        with:
+          node-version-file: '.nvmrc'
+          cache: 'pnpm'
+      - run: pnpm install --frozen-lockfile
+      # Placeholder until the e2e a11y suite (axe-core via Playwright,
+      # per ADR-0016) is wired with the first real screens. The job
+      # exists so branch protection can require it from day one - it
+      # currently no-ops with a clear message.
+      - run: echo "a11y gate placeholder - axe-core via Playwright wires up with the first real screens (ADR-0016)."