fix(ci): replace gitleaks-action with manual install (#50)

## Summary
`gitleaks/gitleaks-action@v2` is now paywalled for organisations — the action requires a `GITLEAKS_LICENSE` secret from gitleaks.io (commercial) or it errors out:

​```
🛑 missing gitleaks license. Go grab one at gitleaks.io and store it as a GitHub Secret named GITLEAKS_LICENSE.
​```

Worse, on Gitea it cannot reliably detect personal-vs-org accounts (different API contract), so it defaults to license enforcement and the scan always fails. The **gitleaks binary itself stays MIT-licensed and free** — only the wrapper went commercial.

Mirror the pattern from #45 (Trivy): drop the wrapper, install the binary directly via curl + tar, run the CLI.

## Scope of the PR
The same two broken integrations existed in `.gitea/workflows/security-scheduled.yml` and would have failed silently at next Monday's cron. Fix both files in one PR for consistency.

- **`ci.yml`** — gitleaks step replaced. Per-PR scan uses `--no-git --source .` (working tree only — the scan job uses a shallow checkout anyway).
- **`security-scheduled.yml`** — both gitleaks AND trivy steps replaced; `fetch-depth: 0` added so gitleaks can do its **deep history scan** here (the value-add of the scheduled job over the per-PR gate); `cache: 'pnpm'` dropped from `actions/setup-node` (consistency with #8 — the act_runner cache server is unreachable from job containers).
- `--redact` on both gitleaks invocations so any matched secret is masked in the CI log itself (avoids re-leaking via log artefacts).

## Trade-off
Like Trivy, gitleaks version is now manually pinned. Same comment in the workflow points to releases for bumps.

## Test plan
- [ ] `scan` job goes green end-to-end on this PR (audit ✓, Trivy ✓, gitleaks ✓).
- [ ] `gitleaks version` line in the install step's logs shows `8.21.0`.
- [ ] On `push` to main post-merge, scan stays green.
- [ ] Trigger `security-scheduled` manually (Actions → Run workflow) to verify the deep-history scan path before next Monday's cron fires.

---------

Co-authored-by: Julien Gautier <julien.gautier@apf.asso.fr>
Reviewed-on: #50

.gitea/workflows/ci.yml

@@ -116,10 +116,40 @@ jobs:
             --exit-code 1 \
             --severity CRITICAL,HIGH \
             .
-      # Secret scan, same reasoning (gitleaks is a Go binary).
-      - uses: gitleaks/gitleaks-action@v2
+      # Secret scan. Same install pattern as Trivy: gitleaks is a Go
+      # binary, and the official `gitleaks/gitleaks-action@v2` wrapper
+      # is now paywalled for organisations (a GITLEAKS_LICENSE secret
+      # from gitleaks.io is required, otherwise the action errors out
+      # with `🛑 missing gitleaks license`). The binary itself stays
+      # MIT-licensed and free — installing it directly bypasses the
+      # wrapper and gives us version pinning for free.
+      - name: Install gitleaks
         env:
-          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          # Bump deliberately. Same caveat as TRIVY_VERSION above —
+          # not Renovate-tracked out of the box. Releases:
+          # https://github.com/gitleaks/gitleaks/releases.
+          GITLEAKS_VERSION: '8.21.0'
+          GITHUB_TOKEN: ${{ secrets.GITHUBCOM_TOKEN }}
+        run: |
+          curl -sfL \
+            -H "Authorization: Bearer ${GITHUB_TOKEN}" \
+            -o /tmp/gitleaks.tar.gz \
+            "https://github.com/gitleaks/gitleaks/releases/download/v${GITLEAKS_VERSION}/gitleaks_${GITLEAKS_VERSION}_linux_x64.tar.gz"
+          tar -xzf /tmp/gitleaks.tar.gz -C /usr/local/bin gitleaks
+          gitleaks version
+      - name: Run gitleaks
+        # `--no-git --source .` scans the working tree only. The scan
+        # job uses a shallow checkout, so a git-history scan would not
+        # see beyond HEAD anyway; the weekly security-scheduled
+        # workflow does the deep history scan with a full clone.
+        # `--redact` masks any matched secret in the log output so we
+        # do not leak it via the CI logs themselves.
+        run: |
+          gitleaks detect \
+            --no-git \
+            --source . \
+            --redact \
+            --exit-code 1
 
   commits:
     # PRs opened by Renovate (apf-portal-bot) carry commit messages