feat: bundle yt-dlp and ffmpeg Linux binaries in bin/

Adds precompiled Linux x86_64 binaries for yt-dlp and ffmpeg under bin/.
The processor resolves yt-dlp from bin/ when present, falling back to PATH.
ffmpeg is passed via --ffmpeg-location so yt-dlp uses the bundled binary on o2switch.
BIN_DIR is configurable via env var.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/lib/ytdlp.ts

@@ -1,3 +1,4 @@
+import { existsSync } from 'fs'
 import path from 'path'
 import { config } from '../../config/app.config'
 
@@ -16,6 +17,11 @@ export function buildYtdlpArgs(params: YtdlpParams): string[] {
 
   const args: string[] = ['--no-playlist', '-o', outputTemplate]
 
+  const ffmpegBin = path.join(config.BIN_DIR, 'ffmpeg')
+  if (existsSync(ffmpegBin)) {
+    args.push('--ffmpeg-location', ffmpegBin)
+  }
+
   if (quality !== 'best') {
     const height = quality.replace('p', '')
     args.push('-f', `${format}[height<=?${height}]+bestaudio/best[height<=?${height}]`)