fix: Windows-safe DB_PATH via env var, add Windows installer scripts

build_windows.bat

@@ -0,0 +1,45 @@
+@echo off
+title BatchTube Windows Build
+echo.
+echo  ============================================================
+echo   BatchTube Windows Installer Builder v1.0.0
+echo   Intentures LLC - https://batchtube.com
+echo  ============================================================
+echo.
+
+python --version >nul 2>&1
+if errorlevel 1 (echo ERROR: Python not found. Install Python 3.11+ && pause && exit /b 1)
+
+echo [1/5] Creating virtual environment...
+if exist build_venv rmdir /s /q build_venv
+python -m venv build_venv
+call build_venv\Scripts\activate.bat
+
+echo [2/5] Installing dependencies...
+pip install --upgrade pip -q
+pip install -r backend\requirements.txt -q
+pip install pyinstaller -q
+
+echo [3/5] Downloading yt-dlp...
+if not exist tools mkdir tools
+curl -L https://github.com/yt-dlp/yt-dlp/releases/latest/download/yt-dlp.exe -o tools\yt-dlp.exe
+
+echo [4/5] Building with PyInstaller...
+cd backend
+pyinstaller batchtube-windows.spec --clean --noconfirm
+cd ..
+
+if not exist backend\dist\BatchTube.exe (echo ERROR: Build failed && pause && exit /b 1)
+echo Build successful: backend\dist\BatchTube.exe
+
+echo [5/5] Creating NSIS installer...
+makensis /VERSION >nul 2>&1
+if errorlevel 1 (
+  echo NSIS not found - install from https://nsis.sourceforge.io/
+  echo Your EXE is ready at: backend\dist\BatchTube.exe
+) else (
+  makensis installer.nsi
+  echo.
+  echo  SUCCESS: BatchTube-Setup-1.0.0.exe created
+)
+pause