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.

cli-config.yaml.example

@@ -684,7 +684,11 @@ platform_toolsets:
 stt:
   enabled: true
   # provider: "local"          # auto-detected if omitted
-  model: "whisper-1"  # whisper-1 (cheapest) | gpt-4o-mini-transcribe | gpt-4o-transcribe
+  local:
+    model: "base"              # tiny | base | small | medium | large-v3 | turbo
+    # language: ""             # auto-detect; set to "en", "es", "fr", etc. to force
+  openai:
+    model: "whisper-1"         # whisper-1 | gpt-4o-mini-transcribe | gpt-4o-transcribe
   # mistral:
   #   model: "voxtral-mini-latest"  # voxtral-mini-latest | voxtral-mini-2602