docs: add samples/ bench dataset spec (KO+EN) + broaden audio gitignore

Document the exact format for the KO+EN labeled clips that the bench gate
needs (manifest.jsonl + ground-truth text + optional entities). Ignore
audio/video under samples/** while keeping manifests tracked.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -21,8 +21,12 @@ venv/
 # Models / data / scratch
 *.log
 models/
-samples/*.wav
-samples/*.mp4
+samples/**/*.wav
+samples/**/*.flac
+samples/**/*.mp3
+samples/**/*.m4a
+samples/**/*.mp4
+samples/**/*.mov
 
 # ─── OS / editor ──────────────────────────────────────────
 .DS_Store