nutrition: pre-download HuggingFace models at build time

The deployment server couldn't fetch transformer models at runtime due to
restricted network access and permission errors writing to node_modules.
Add a prebuild script to download models during build and document
TRANSFORMERS_CACHE env var for configuring a shared writable cache path.

.env.example

@@ -33,3 +33,6 @@ DEEPL_API_URL="https://api-free.deepl.com/v2/translate"  # Use https://api.deepl
 
 # AI Vision Service (Ollama for Alt Text Generation)
 OLLAMA_URL="http://localhost:11434"  # Local Ollama server URL
+
+# HuggingFace Transformers Model Cache (for nutrition embedding models)
+TRANSFORMERS_CACHE="/var/cache/transformers"  # Must be writable by build and runtime user