refactor: unify voice transcription config across all channels

- Move transcriptionProvider to global channels config (not per-channel)
- ChannelManager auto-resolves API key from matching provider config
- BaseChannel gets transcription_provider attribute, no more getattr hack
- Remove redundant transcription fields from WhatsAppConfig
- Update README: document transcriptionProvider, update provider table

Made-with: Cursor

nanobot/channels/manager.py

@@ -39,7 +39,8 @@ class ChannelManager:
         """Initialize channels discovered via pkgutil scan + entry_points plugins."""
         from nanobot.channels.registry import discover_all
 
-        groq_key = self.config.providers.groq.api_key
+        transcription_provider = self.config.channels.transcription_provider
+        transcription_key = self._resolve_transcription_key(transcription_provider)
 
         for name, cls in discover_all().items():
             section = getattr(self.config.channels, name, None)
@@ -54,7 +55,8 @@ class ChannelManager:
                 continue
             try:
                 channel = cls(section, self.bus)
-                channel.transcription_api_key = groq_key
+                channel.transcription_provider = transcription_provider
+                channel.transcription_api_key = transcription_key
                 self.channels[name] = channel
                 logger.info("{} channel enabled", cls.display_name)
             except Exception as e:
@@ -62,6 +64,12 @@ class ChannelManager:
 
         self._validate_allow_from()
 
+    def _resolve_transcription_key(self, provider: str) -> str:
+        """Pick the API key for the configured transcription provider."""
+        if provider == "openai":
+            return self.config.providers.openai.api_key
+        return self.config.providers.groq.api_key
+
     def _validate_allow_from(self) -> None:
         for name, ch in self.channels.items():
             if getattr(ch.config, "allow_from", None) == []: