ci: add Windows build workflow (PyInstaller + NSIS)

- Triggers on version tags (v*) or workflow_dispatch
- Builds BatchTube.exe via PyInstaller
- Creates BatchTube-Setup-*.exe via NSIS
- Uploads artifacts for download

.github/workflows/build-windows.yml

@@ -0,0 +1,66 @@
+name: BatchTube Windows Build
+
+on:
+  push:
+    tags:
+      - 'v*'
+  workflow_dispatch:
+    inputs:
+      version:
+        description: 'Version tag (e.g. v1.0.0)'
+        required: false
+        default: 'v1.0.0'
+
+jobs:
+  build-windows:
+    runs-on: windows-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup Python 3.11
+        uses: actions/setup-python@v5
+        with:
+          python-version: '3.11'
+
+      - name: Install dependencies
+        run: |
+          cd backend
+          pip install --upgrade pip
+          pip install -r requirements.txt
+          pip install pyinstaller
+
+      - name: Download yt-dlp.exe
+        run: |
+          mkdir -p tools
+          curl -L https://github.com/yt-dlp/yt-dlp/releases/latest/download/yt-dlp.exe -o tools/yt-dlp.exe
+
+      - name: Create empty database
+        run: |
+          cd backend
+          python -c "import sqlite3; sqlite3.connect('youtube_sipoc.db').close()"
+
+      - name: Build Windows EXE
+        run: |
+          cd backend
+          pyinstaller batchtube-windows.spec --clean --noconfirm
+
+      - name: Install NSIS
+        run: choco install nsis -y
+
+      - name: Build NSIS Installer
+        run: makensis installer.nsi
+
+      - name: Upload EXE artifact
+        uses: actions/upload-artifact@v4
+        with:
+          name: BatchTube-Windows-EXE
+          path: backend/dist/BatchTube.exe
+          retention-days: 30
+
+      - name: Upload Installer artifact
+        uses: actions/upload-artifact@v4
+        with:
+          name: BatchTube-Windows-Installer
+          path: BatchTube-Setup-*.exe
+          retention-days: 30