chore(ci): drop unused GitHub Actions backend workflow

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

.github/workflows/backend-ci.yml

@@ -1,56 +0,0 @@
-name: Backend CI
-
-on:
-  push:
-    paths:
-      - "server/**"
-      - ".github/workflows/backend-ci.yml"
-  pull_request:
-    paths:
-      - "server/**"
-      - ".github/workflows/backend-ci.yml"
-
-permissions:
-  contents: read
-
-jobs:
-  backend:
-    runs-on: ubuntu-latest
-    defaults:
-      run:
-        working-directory: server
-
-    steps:
-      - name: Checkout
-        uses: actions/checkout@v4
-
-      - name: Setup Go
-        uses: actions/setup-go@v5
-        with:
-          go-version-file: server/go.mod
-          cache-dependency-path: server/go.sum
-
-      - name: Add Go bin to PATH
-        run: echo "$(go env GOPATH)/bin" >> "$GITHUB_PATH"
-
-      - name: Install sqlc
-        run: go install github.com/sqlc-dev/sqlc/cmd/sqlc@v1.30.0
-
-      - name: Generate sqlc code
-        run: make sqlc-generate
-
-      - name: Verify generated code is committed
-        run: git diff --exit-code -- internal/tenant/repository/generated
-
-      - name: Check tenant SQL scope guard
-        run: make tenant-scope-guard
-
-      - name: Run tests
-        run: make test
-
-      - name: Run golangci-lint
-        uses: golangci/golangci-lint-action@v6
-        with:
-          version: v1.59.1
-          working-directory: server
-          args: ./...