fix: STT provider-model mismatch — whisper-1 fed to faster-whisper (#7113)

Legacy flat stt.model config key (from cli-config.yaml.example and older
versions) was passed as a model override to transcribe_audio() by the
gateway, bypassing provider-specific model resolution. When the provider
was 'local' (faster-whisper), this caused:
  ValueError: Invalid model size 'whisper-1'

Changes:
- gateway/run.py, discord.py: stop passing model override — let
  transcribe_audio() handle provider-specific model resolution internally
- get_stt_model_from_config(): now provider-aware, reads from the correct
  nested section (stt.local.model, stt.openai.model, etc.); ignores
  legacy flat key for local provider to prevent model name mismatch
- cli-config.yaml.example: updated STT section to show nested provider
  config structure instead of legacy flat key
- config migration v13→v14: moves legacy stt.model to the correct
  provider section and removes the flat key

Reported by community user on Discord.

gateway/platforms/discord.py

@@ -1260,9 +1260,8 @@ class DiscordAdapter(BasePlatformAdapter):
         try:
             await asyncio.to_thread(VoiceReceiver.pcm_to_wav, pcm_data, wav_path)
 
-            from tools.transcription_tools import transcribe_audio, get_stt_model_from_config
-            stt_model = get_stt_model_from_config()
-            result = await asyncio.to_thread(transcribe_audio, wav_path, model=stt_model)
+            from tools.transcription_tools import transcribe_audio
+            result = await asyncio.to_thread(transcribe_audio, wav_path)
 
             if not result.get("success"):
                 return