fix(reinstall): use OSTemplate.vmid (not template_vmid) and convert memory bytes->MB

- AttributeError: OSTemplate has no 'template_vmid' field; the actual column is 'vmid'.
- Use OSTemplate.get_vmid_for_node() to support cross-node replicated templates,
  falling back to source_node/vmid otherwise.
- VMInstance.memory is stored in bytes, but Proxmox config expects MB.

.github/workflows/backend-tests.yml

@@ -0,0 +1,57 @@
+name: Backend Tests
+
+on:
+  push:
+    branches: [main]
+    paths:
+      - "backend/**"
+      - ".github/workflows/backend-tests.yml"
+  pull_request:
+    branches: [main]
+    paths:
+      - "backend/**"
+      - ".github/workflows/backend-tests.yml"
+
+jobs:
+  pytest:
+    name: pytest (Python ${{ matrix.python-version }})
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        python-version: ["3.12"]
+
+    defaults:
+      run:
+        working-directory: backend
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Python ${{ matrix.python-version }}
+        uses: actions/setup-python@v5
+        with:
+          python-version: ${{ matrix.python-version }}
+          cache: pip
+          cache-dependency-path: backend/requirements-dev.txt
+
+      - name: Install dependencies
+        run: |
+          python -m pip install --upgrade pip
+          pip install -r requirements-dev.txt
+
+      - name: Run pytest with coverage
+        run: |
+          python -m pytest \
+            --cov=app \
+            --cov-report=term-missing \
+            --cov-report=xml \
+            -p no:warnings
+
+      - name: Upload coverage artifact
+        if: always()
+        uses: actions/upload-artifact@v4
+        with:
+          name: backend-coverage-${{ matrix.python-version }}
+          path: backend/coverage.xml
+          if-no-files-found: ignore