feat(whatsapp): add voice message transcription via OpenAI/Groq Whisper

Automatically transcribe WhatsApp voice messages using OpenAI Whisper
or Groq. Configurable via transcriptionProvider and transcriptionApiKey.

Config:
  "whatsapp": {
    "transcriptionProvider": "openai",
    "transcriptionApiKey": "sk-..."
  }

nanobot/providers/transcription.py

@@ -1,8 +1,36 @@
-"""Voice transcription provider using Groq."""
+"""Voice transcription providers (Groq and OpenAI Whisper)."""
 
 import os
 from pathlib import Path
 
+
+class OpenAITranscriptionProvider:
+    """Voice transcription provider using OpenAI's Whisper API."""
+
+    def __init__(self, api_key: str | None = None):
+        self.api_key = api_key or os.environ.get("OPENAI_API_KEY")
+        self.api_url = "https://api.openai.com/v1/audio/transcriptions"
+
+    async def transcribe(self, file_path: str | Path) -> str:
+        if not self.api_key:
+            return ""
+        path = Path(file_path)
+        if not path.exists():
+            return ""
+        try:
+            import httpx
+            async with httpx.AsyncClient() as client:
+                with open(path, "rb") as f:
+                    files = {"file": (path.name, f), "model": (None, "whisper-1")}
+                    headers = {"Authorization": f"Bearer {self.api_key}"}
+                    response = await client.post(
+                        self.api_url, headers=headers, files=files, timeout=60.0,
+                    )
+                    response.raise_for_status()
+                    return response.json().get("text", "")
+        except Exception:
+            return ""
+
 import httpx
 from loguru import logger