Fix cosyvoice models not showing on model page and runtime import errors

- Model filter in ModelManagement was an allowlist missing cosyvoice prefix,
  replaced with inverse filter (everything except whisper = voice model)
- Shim training-only modules (cosyvoice.dataset.processor, matcha.utils.*)
  to prevent hyperpyyaml from importing pyarrow, pyworld, lightning etc.
- Patch torchaudio.load for 2.9+ compat (torchcodec now required by default)
- Add matplotlib to requirements (matcha hifigan imports it at module level)

backend/requirements.txt

@@ -48,6 +48,7 @@ openai-whisper>=20231117
 tiktoken
 einops
 inflect
+matplotlib
 
 # Audio processing
 librosa>=0.10.0