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

README.md

@@ -900,7 +900,7 @@ IMAP_PASSWORD=your-password-here
 ### Providers
 
 > [!TIP]
-> - **Groq** provides free voice transcription via Whisper. If configured, Telegram voice messages will be automatically transcribed.
+> - **Voice transcription**: Voice messages (Telegram, WhatsApp) are automatically transcribed using Whisper. By default Groq is used (free tier). Set `"transcriptionProvider": "openai"` under `channels` to use OpenAI Whisper instead — the API key is picked from the matching provider config.
 > - **MiniMax Coding Plan**: Exclusive discount links for the nanobot community: [Overseas](https://platform.minimax.io/subscribe/coding-plan?code=9txpdXw04g&source=link) · [Mainland China](https://platform.minimaxi.com/subscribe/token-plan?code=GILTJpMTqZ&source=link)
 > - **MiniMax (Mainland China)**: If your API key is from MiniMax's mainland China platform (minimaxi.com), set `"apiBase": "https://api.minimaxi.com/v1"` in your minimax provider config.
 > - **VolcEngine / BytePlus Coding Plan**: Use dedicated providers `volcengineCodingPlan` or `byteplusCodingPlan` instead of the pay-per-use `volcengine` / `byteplus` providers.
@@ -916,9 +916,9 @@ IMAP_PASSWORD=your-password-here
 | `byteplus` | LLM (VolcEngine international, pay-per-use) | [Coding Plan](https://www.byteplus.com/en/activity/codingplan?utm_campaign=nanobot&utm_content=nanobot&utm_medium=devrel&utm_source=OWO&utm_term=nanobot) · [byteplus.com](https://www.byteplus.com) |
 | `anthropic` | LLM (Claude direct) | [console.anthropic.com](https://console.anthropic.com) |
 | `azure_openai` | LLM (Azure OpenAI) | [portal.azure.com](https://portal.azure.com) |
-| `openai` | LLM (GPT direct) | [platform.openai.com](https://platform.openai.com) |
+| `openai` | LLM + Voice transcription (Whisper) | [platform.openai.com](https://platform.openai.com) |
 | `deepseek` | LLM (DeepSeek direct) | [platform.deepseek.com](https://platform.deepseek.com) |
-| `groq` | LLM + **Voice transcription** (Whisper) | [console.groq.com](https://console.groq.com) |
+| `groq` | LLM + Voice transcription (Whisper, default) | [console.groq.com](https://console.groq.com) |
 | `minimax` | LLM (MiniMax direct) | [platform.minimaxi.com](https://platform.minimaxi.com) |
 | `gemini` | LLM (Gemini direct) | [aistudio.google.com](https://aistudio.google.com) |
 | `aihubmix` | LLM (API gateway, access to all models) | [aihubmix.com](https://aihubmix.com) |
@@ -1233,6 +1233,7 @@ Global settings that apply to all channels. Configure under the `channels` secti
     "sendProgress": true,
     "sendToolHints": false,
     "sendMaxRetries": 3,
+    "transcriptionProvider": "groq",
     "telegram": { ... }
   }
 }
@@ -1243,6 +1244,7 @@ Global settings that apply to all channels. Configure under the `channels` secti
 | `sendProgress` | `true` | Stream agent's text progress to the channel |
 | `sendToolHints` | `false` | Stream tool-call hints (e.g. `read_file("…")`) |
 | `sendMaxRetries` | `3` | Max delivery attempts per outbound message, including the initial send (0-10 configured, minimum 1 actual attempt) |
+| `transcriptionProvider` | `"groq"` | Voice transcription backend: `"groq"` (free tier, default) or `"openai"`. API key is auto-resolved from the matching provider config. |
 
 #### Retry Behavior