Fix: Prevent crashes when HuggingFace is unreachable

Implements offline mode patch for API stability issues:

- Add hf_offline_patch.py to monkey-patch huggingface_hub
- Force cache-only lookups before mlx_audio imports
- Create symlink from original Qwen repo to MLX community version
  when only MLX version is cached

This fixes:
- Issue #150: Internet required even with cached models
- Issue #151: API crashes when HF network fails

The patch ensures that if models are locally cached, no network
requests are made to HuggingFace during speech generation.

PATCH_NOTES.md

@@ -0,0 +1,58 @@
+# Voicebox Offline Mode Fix
+
+## Problem
+Voicebox crashes when generating speech if HuggingFace is unreachable, even when models are fully cached locally.
+
+**Root Cause:**
+- Voicebox downloads `mlx-community/Qwen3-TTS-12Hz-1.7B-Base-bf16` (MLX optimized version)
+- But `mlx_audio.tts.load()` tries to fetch `config.json` from original repo `Qwen/Qwen3-TTS-12Hz-1.7B-Base`
+- This network request fails → server crashes with `RemoteDisconnected`
+
+**Related Issues:**
+- Issue #150: "Internet connection required, even though models are downloaded?"
+- Issue #151: "API Stability Issues: Model Loading Hangs and Server Crashes"
+
+## Solution
+Two-part fix:
+
+### 1. Monkey-patch huggingface_hub (`backend/utils/hf_offline_patch.py`)
+- Intercepts cache lookup functions
+- Forces offline mode early (before mlx_audio imports)
+- Adds debug logging for cache hits/misses
+
+### 2. Symlink original repo to MLX version (`ensure_original_qwen_config_cached()`)
+- When original `Qwen/Qwen3-TTS-12Hz-1.7B-Base` cache doesn't exist
+- But MLX `mlx-community/Qwen3-TTS-12Hz-1.7B-Base-bf16` does exist
+- Creates a symlink so cache lookups succeed
+
+## Files Changed
+- `backend/backends/mlx_backend.py` - Added patch imports at top
+- `backend/utils/hf_offline_patch.py` - New patch module
+
+## Testing
+To test this fix:
+1. Build Voicebox from source: `make build`
+2. Disconnect from internet
+3. Try generating speech
+4. Should work without network requests
+
+## Build Instructions
+
+```bash
+# Install dependencies
+pip install -r requirements.txt
+
+# Build the app
+make build
+
+# Or build just the server
+make build-server
+```
+
+## Notes
+- The patch is applied automatically when `mlx_backend.py` is imported
+- Set `VOICEBOX_OFFLINE_PATCH=0` to disable the patch
+- The symlink approach works because the config.json is compatible between versions
+
+---
+*Patch contributed by community*