docs(devops): document two-token model, OSV alerts, and nightly-audit runbook in ci-gitea.md

Adds §Renovate + Nightly Audit — Token Model covering:
- Two-token model: RENOVATE_TOKEN vs NIGHTLY_AUDIT_TOKEN (issues-only), blast
  radius rationale, PAT rotation cadence (annual + on-compromise)
- OSV vs platform alerts on Gitea — osvVulnerabilityAlerts is the load-bearing
  detector; Gitea exposes no vulnerability graph for vulnerabilityAlerts
- Nightly vs PR gate divergence table (dev deps in/out)
- Runbook: triage severity → pin/upgrade/override → close issue

Refs #818. See ADR-041 for full rationale.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/infrastructure/ci-gitea.md

@@ -462,3 +462,82 @@ jobs:
           name: e2e-results
           path: frontend/test-results/e2e/
 ```
+
+---
+
+## Renovate + Nightly Audit — Token Model
+
+> See [ADR-041](../adr/041-renovate-runner-setup.md) for full rationale.
+
+### Two-token model
+
+This repo uses two separate tokens for automated dependency work, both manually
+provisioned as Gitea secrets. There is **no auto-provided `GITEA_TOKEN`** on
+self-hosted Gitea runners — it must be created manually (see §Context Variable Names
+table above).
+
+| Secret | Scopes | Job | Reason |
+|--------|--------|-----|--------|
+| `RENOVATE_TOKEN` | `contents` + `pull_request` + `issues` | `renovate.yml` | Renovate needs to read/write files and open PRs |
+| `NIGHTLY_AUDIT_TOKEN` | `issues` only | `nightly.yml` → `npm-audit` job | Only needs to file a tracking issue; an `issues`-only token cannot push branches or read contents — limits blast radius on leak |
+
+Both tokens belong to a single dedicated bot account. **Branch protection on `main`
+must forbid the bot pushing directly**, because a `contents`-scoped token can push
+to any unprotected branch.
+
+### PAT rotation cadence
+
+Rotate both tokens:
+- **Annually** (calendar reminder)
+- **Immediately** on suspected compromise (runner log leak, accidental `set -x`, etc.)
+
+Tokens are stored exclusively as Gitea secrets. Never commit them to `.env` files or
+log them. The nightly audit step passes its token via `env:` at the step level, reads
+it as `$NIGHTLY_AUDIT_TOKEN` in the shell, and never runs the API `curl` under
+`set -x`.
+
+### OSV vs platform alerts on Gitea
+
+Renovate's `vulnerabilityAlerts` config key requires a *platform* vulnerability graph.
+**Gitea does not expose one** — it has no equivalent to the GitHub Advisory Database
+API. On this runner, `vulnerabilityAlerts` is a label carrier only: it attaches
+`security` and `P1-high` labels to PRs that `osvVulnerabilityAlerts` already raised.
+
+`osvVulnerabilityAlerts: true` is the load-bearing detector. Renovate queries
+[OSV.dev](https://osv.dev) directly, which works regardless of platform. The runner
+host must be able to reach `osv.dev:443`. If egress is filtered and OSV.dev is
+unreachable, the flag silently no-ops — verify egress when standing up the runner.
+
+### Nightly audit vs PR gate (divergence)
+
+| Gate | Command | Dev deps | When |
+|------|---------|----------|------|
+| PR gate (`ci.yml`) | `npm audit --audit-level=high --omit=dev` | ❌ excluded | Every PR |
+| Nightly audit (`nightly.yml`) | `npm audit --audit-level=high` | ✅ included | Nightly + `workflow_dispatch` |
+
+The nightly job is **deliberately broader** — it catches dev-tooling advisories
+(esbuild, Vite, vitest, etc.) that the PR gate ignores. A red nightly audit job does
+**not** mean the PR gate is broken; the two signals are independent.
+
+### Runbook: nightly-opened tracking issue
+
+When the `npm-audit` job opens or updates the tracking issue
+"Nightly npm audit: high-severity advisory":
+
+1. **Triage severity.** Check the advisory page (link in the issue body). Is it
+   exploitable in production? A dev-only dep (e.g. esbuild, prettier) has no
+   production attack surface — treat as low urgency.
+
+2. **Pin or upgrade.** If a non-breaking upgrade is available, update
+   `frontend/package.json` and regenerate the lockfile. Open a PR.
+
+3. **Override if justified.** If the advisory does not apply (dev-only dep, no
+   exploitable path), add an `npm audit` override in `package.json`:
+   ```json
+   "overrides": { "esbuild": ">=0.25.4" }
+   ```
+   Document the rationale in the PR body. See #817 for the reference decision tree.
+
+4. **Close the tracking issue** once the advisory is resolved or overridden and the
+   nightly job runs clean (verify via the `✅ npm audit clean` heartbeat in the job
+   summary).