simplify: remove transcription fallback, respect explicit config

Configured provider is the only one used — no silent fallback.

Made-with: Cursor

nanobot/channels/manager.py

@@ -41,8 +41,6 @@ class ChannelManager:
 
         transcription_provider = self.config.channels.transcription_provider
         transcription_key = self._resolve_transcription_key(transcription_provider)
-        fallback_provider = "groq" if transcription_provider == "openai" else "openai"
-        fallback_key = self._resolve_transcription_key(fallback_provider)
 
         for name, cls in discover_all().items():
             section = getattr(self.config.channels, name, None)
@@ -59,7 +57,6 @@ class ChannelManager:
                 channel = cls(section, self.bus)
                 channel.transcription_provider = transcription_provider
                 channel.transcription_api_key = transcription_key
-                channel._transcription_fallback_key = fallback_key
                 self.channels[name] = channel
                 logger.info("{} channel enabled", cls.display_name)
             except Exception as e: