docs: update audio config docs and add voice message failure fix to changelog

- README.md: Update audio config format to match schema (enabled + provider.* fields instead of old transcription_endpoint fields), add whisper.cpp server Docker example
- CHANGELOG.md: Add '### Fixed' section with voice message failure handling details
- config/default.yaml: Update audio section with new schema format and Docker setup example

README.md

@@ -170,20 +170,37 @@ Configure a Whisper-compatible endpoint for models that don't support native aud
 
 ```yaml
 audio:
-  transcription_endpoint: "http://localhost:8080/v1/audio/transcriptions"
-  transcription_api_key: "${WHISPER_API_KEY}"    # Optional Bearer token
-  transcription_model: "whisper-1"               # Model name (default: whisper-1)
-  transcription_provider: "openai"               # Provider format: openai (default)
+  enabled: true
+  provider:
+    type: custom                            # openai, groq, ollama, llamacpp, custom
+    endpoint: "http://localhost:18801/v1/audio/transcriptions"
+    api_key: "${WHISPER_API_KEY}"            # Optional Bearer token
+    model: "whisper-1"                       # Model name (default: whisper-1)
 ```
 
 | Field | Required | Description |
 |-------|----------|-------------|
-| `transcription_endpoint` | yes | Whisper-compatible API endpoint |
-| `transcription_api_key` | no | Bearer token for authentication |
-| `transcription_model` | no | Model name sent in the request (default: `whisper-1`) |
-| `transcription_provider` | no | API format: `openai` (default) |
+| `enabled` | no | Enable audio transcription (default: `false`) |
+| `provider.type` | yes | Provider type: `openai`, `groq`, `ollama`, `llamacpp`, or `custom` |
+| `provider.endpoint` | yes | Whisper-compatible API endpoint |
+| `provider.api_key` | no | Bearer token for authentication |
+| `provider.model` | no | Model name sent in request (default: `whisper-1`) |
 
-Without an `audio` config, voice messages from non-audio-capable models are silently skipped.
+Without an `audio` config, voice messages from non-audio-capable models will display an error message to the user. For local transcription, you can run a whisper.cpp server:
+
+```bash
+# Start whisper.cpp server with OpenAI-compatible endpoint
+docker run -d \
+  --name whisper-server \
+  -p 18801:8080 \
+  ghcr.io/ggml-org/whisper.cpp:main \
+  --model /app/models/ggml-base.en.bin \
+  --host 0.0.0.0 \
+  --port 8080 \
+  --convert \
+  --language en \
+  --inference-path /v1/audio/transcriptions
+```
 
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