chore(audio): generate Vietnamese clips for both voices

- Run scripts/generate-audio.py to produce 184 MP3 clips (92 each
  for vi-VN-HoaiMyNeural and vi-VN-NamMinhNeural), ~2.2 MB total.
- Cap edge-tts concurrency at 4 with 4-attempt exponential retry on
  NoAudioReceived — earlier all-at-once gather() hit the upstream
  rate limit and bailed mid-voice.
- .gitignore: add .venv/ + __pycache__/ for the local generator venv.

web/static/audio/manifest.json

@@ -13,4 +13,4 @@
       "gender": "male"
     }
   ]
-}
+}