ci: add custom CodeQL workflow for backend/ Go module

.github/workflows/codeql.yml

@@ -0,0 +1,65 @@
+name: CodeQL
+
+on:
+  push:
+    branches: [main, dev]
+  pull_request:
+    branches: [main, dev]
+  schedule:
+    - cron: "23 5 * * 1"
+
+jobs:
+  analyze:
+    name: Analyze (${{ matrix.language }})
+    runs-on: ubuntu-latest
+    permissions:
+      actions: read
+      contents: read
+      security-events: write
+
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - language: go
+            build-mode: manual
+          - language: javascript-typescript
+            build-mode: none
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v5
+
+      - name: Set up Go
+        if: matrix.language == 'go'
+        uses: actions/setup-go@v6
+        with:
+          go-version: "1.25"
+
+      - name: Install swag
+        if: matrix.language == 'go'
+        run: go install github.com/swaggo/swag/cmd/swag@latest
+
+      - name: Generate Swagger docs
+        if: matrix.language == 'go'
+        working-directory: backend
+        run: swag init -d cmd/server,internal/api -g main.go --parseDependency --parseInternal -o docs
+
+      - name: Initialize CodeQL
+        uses: github/codeql-action/init@v3
+        with:
+          languages: ${{ matrix.language }}
+          build-mode: ${{ matrix.build-mode }}
+
+      # The Go module lives in backend/ and its swagger docs package is
+      # generated above. Autobuild cannot locate the module from the repo
+      # root, so build explicitly from backend/ here.
+      - name: Build Go
+        if: matrix.language == 'go'
+        working-directory: backend
+        run: go build ./...
+
+      - name: Perform CodeQL Analysis
+        uses: github/codeql-action/analyze@v3
+        with:
+          category: "/language:${{ matrix.language }}"