docs: add HuggingFace cache troubleshooting to README

- Document HF_HOME environment variable for writable cache
- Add systemd service permission guidance for /tmp paths
- Troubleshooting steps for read-only file system errors

.env.testing

@@ -16,3 +16,6 @@ VOICES_DIR=./voices
 # Default voice name (optional - uses first found voice if not set)
 # This should match the filename without .wav extension (case-insensitive)
 # DEFAULT_VOICE=masterchief
+
+# HuggingFace cache directory (must be writable)
+HF_HOME=/tmp/huggingface