ci: add Go build/test and gitleaks secret-scan workflows

- go.yml: gofmt check, build, vet, and race-enabled tests for backend/,
  triggered on backend changes and pushes to main.
- gitleaks.yml: secret scanning on PRs and main using gitleaks-action v1
  (license-free; v2 requires GITLEAKS_LICENSE for org repos).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/workflows/gitleaks.yml

@@ -0,0 +1,22 @@
+name: gitleaks
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+permissions:
+  contents: read
+
+jobs:
+  scan:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      # gitleaks-action v1 scans for committed secrets and needs no license
+      # key (v2 requires GITLEAKS_LICENSE for organization repos).
+      - name: Scan for secrets
+        uses: zricethezav/gitleaks-action@v1.6.0

.github/workflows/go.yml

@@ -0,0 +1,44 @@
+name: Go
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    paths:
+      - "backend/**"
+      - ".github/workflows/go.yml"
+
+permissions:
+  contents: read
+
+jobs:
+  build-test:
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: backend
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-go@v5
+        with:
+          go-version: "1.22"
+          cache: false
+
+      - name: Check formatting
+        run: |
+          unformatted=$(gofmt -l .)
+          if [ -n "$unformatted" ]; then
+            echo "These files need gofmt:"
+            echo "$unformatted"
+            exit 1
+          fi
+
+      - name: Build
+        run: go build ./...
+
+      - name: Vet
+        run: go vet ./...
+
+      - name: Test
+        run: go test -race ./...