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/config/schema.py

@@ -28,6 +28,7 @@ class ChannelsConfig(Base):
     send_progress: bool = True  # stream agent's text progress to the channel
     send_tool_hints: bool = False  # stream tool-call hints (e.g. read_file("…"))
     send_max_retries: int = Field(default=3, ge=0, le=10)  # Max delivery attempts (initial send included)
+    transcription_provider: str = "groq"  # Voice transcription backend: "groq" or "openai"
 
 
 class DreamConfig(Base):