Add a pinned secret-scanning workflow

Scans the full git history on push, pull request and weekly, so a credential
that was committed and later removed is still detected. The scanner archive is
pinned by version and verified by SHA-256 before it runs, and matched values
are redacted so they never reach the CI log.

.gitea/workflows/secret-scan.yml

@@ -0,0 +1,47 @@
+name: secret-scan
+
+# Scans the FULL git history, not just the working tree: a credential that was
+# committed and later deleted is still published, and only a history scan sees it.
+# The weekly run exists because new detector rules find old secrets that passed
+# when they were first pushed.
+
+on:
+  push:
+  pull_request:
+  schedule:
+    - cron: "0 5 * * 1"
+
+jobs:
+  gitleaks:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out full history
+        # Pinned to a commit, not a mutable tag: a security job must not silently
+        # change what it runs when upstream retags.
+        uses: actions/checkout@11d5960a326750d5838078e36cf38b85af677262 # v4
+        with:
+          fetch-depth: 0
+
+      - name: Install pinned Gitleaks
+        env:
+          GITLEAKS_VERSION: "8.30.1"
+          GITLEAKS_SHA256: "551f6fc83ea457d62a0d98237cbad105af8d557003051f41f3e7ca7b3f2470eb"
+        run: |
+          set -euo pipefail
+          archive="gitleaks_${GITLEAKS_VERSION}_linux_x64.tar.gz"
+          curl -fsSL -o "${archive}" \
+            "https://github.com/gitleaks/gitleaks/releases/download/v${GITLEAKS_VERSION}/${archive}"
+          # Fail loudly on any mismatch rather than scanning with an unverified binary.
+          printf '%s  %s\n' "${GITLEAKS_SHA256}" "${archive}" | sha256sum -c -
+          tar -xzf "${archive}" gitleaks
+          chmod 0755 gitleaks
+          ./gitleaks version
+
+      - name: Scan full history for secrets
+        # --redact=100 keeps matched values out of the CI log, which is itself
+        # readable by anyone who can see the run.
+        run: ./gitleaks git . --redact=100 --no-banner --exit-code 1
+
+      - name: Remove the downloaded scanner
+        if: always()
+        run: rm -f gitleaks gitleaks_*.tar.gz