feat: add Linux ffprobe and Windows ffmpeg/ffprobe binaries to bin/

yt-dlp needs ffprobe alongside ffmpeg for postprocessing; only ffmpeg was
bundled for the Linux/o2switch prod deploy. Also add Windows builds so
local dev on Windows has a working ffmpeg/ffprobe pair, and gitignore
.env.prod/.env.local so real deployment credentials never get committed.

src/lib/ytdlp.ts

@@ -55,7 +55,9 @@ export function buildYtdlpArgs(params: YtdlpParams): string[] {
 
   const args: string[] = ['--no-playlist', '-o', outputTemplate]
 
-  const ffmpegBin = path.join(config.BIN_DIR, 'ffmpeg')
+  // bin/ffmpeg is a Linux ELF binary bundled for o2switch prod deploy; it can't
+  // run on Windows, so local dev instead looks for a bin/ffmpeg.exe.
+  const ffmpegBin = path.join(config.BIN_DIR, process.platform === 'win32' ? 'ffmpeg.exe' : 'ffmpeg')
   if (existsSync(ffmpegBin)) {
     args.push('--ffmpeg-location', ffmpegBin)
   }