fix(ci): run scanners before pnpm install to avoid node_modules false positives

First successful gitleaks run flagged 381 "leaks" — all of them
inside `node_modules/` and `.pnpm-store/`, populated by the
`pnpm install --frozen-lockfile` step that runs earlier in the
job. Upstream packages routinely embed demo RSA keys / fake API
tokens in their READMEs and test fixtures, and gitleaks
(correctly, by its rules) flags them all. This is the same class
of false-positive Trivy hit before us in #49.

Move both scanners (Trivy + gitleaks) to BEFORE `pnpm install`:

- Trivy scans `pnpm-lock.yaml` for vulns; the lockfile is
  committed, no install required.
- Gitleaks scans the working tree (`--no-git --source .` in
  ci.yml; deep history in security-scheduled.yml). Without
  `pnpm install`, the only files present are our own source
  code, which is what we actually want to scan.
- `pnpm audit` reads `pnpm-lock.yaml` against the advisory DB —
  also doesn't need node_modules. The install before audit
  remains for the workspace-integrity sanity check.

Net result: clean scans, no allowlist file to maintain, scanners
run faster (smaller tree to walk).

The ordering rationale is documented inline at the top of each
job's `steps:` block so a future contributor doesn't innocently
shuffle the steps and re-introduce the false-positive flood.

Apply the same reordering to `security-scheduled.yml` for
consistency, even though its deep-history gitleaks scan does not
suffer the same false positives (history does not contain
node_modules; gitignored from day one).

.gitea/workflows/ci.yml

@@ -55,14 +55,22 @@ jobs:
 
   scan:
     runs-on: [self-hosted, on-prem]
+    # Step ordering matters here: Trivy and gitleaks BOTH run before
+    # `pnpm install`. Reason: gitleaks scans the working tree
+    # (`--no-git --source .`), and after install, `node_modules/`
+    # and `.pnpm-store/` are full of upstream packages whose READMEs
+    # and test fixtures contain demo RSA keys / fake API tokens —
+    # gitleaks then false-positives on them by the hundreds (caught
+    # the hard way: 381 hits on the first run). Trivy reads
+    # `pnpm-lock.yaml` for its vuln scan, not `node_modules`, so it
+    # also doesn't need install. `pnpm ci:audit` does the same — it
+    # queries the advisory DB against the lockfile.
     steps:
       - uses: actions/checkout@v6
       - uses: pnpm/action-setup@v6
       - uses: actions/setup-node@v6
         with:
           node-version-file: '.nvmrc'
-      - run: pnpm install --frozen-lockfile
-      - run: pnpm ci:audit
       # Dependency vulnerability scan. Trivy is a Go binary, not an npm
       # package, so it cannot live in package.json scripts as cleanly
       # as audit/lint do.
@@ -150,6 +158,11 @@ jobs:
             --source . \
             --redact \
             --exit-code 1
+      # npm-advisory check (against pnpm-lock.yaml). Run last so
+      # `pnpm install` does not pollute the working tree before the
+      # scanners above.
+      - run: pnpm install --frozen-lockfile
+      - run: pnpm ci:audit
 
   commits:
     # PRs opened by Renovate (apf-portal-bot) carry commit messages