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>

config/app.config.ts

@@ -1,3 +1,5 @@
+import path from 'path'
+
 export const config = {
   DOWNLOAD_LINK_TTL_HOURS: 24,
   WORKER_CONCURRENCY: 3,
@@ -5,4 +7,5 @@ export const config = {
   STORAGE_PATH: process.env.STORAGE_PATH ?? '',
   RATE_LIMIT_MAX: 5,
   RATE_LIMIT_WINDOW_MS: 3_600_000,
+  BIN_DIR: process.env.BIN_DIR ?? path.join(process.cwd(), 'bin'),
 } as const