feat: add product-lookup Edge Function (#24)

- Fetch product data from Open Food Facts API
- Cache results in products table
- Handle product not found gracefully
- CORS enabled for frontend access
- Returns cached data for performance

Closes #24

supabase/functions/product-lookup/README.md

@@ -0,0 +1,83 @@
+# Product Lookup Edge Function
+
+Fetches product data from Open Food Facts API by barcode and caches results in the database.
+
+## Endpoint
+
+`POST /functions/v1/product-lookup`
+
+## Request
+
+```json
+{
+  "barcode": "8000500310427"
+}
+```
+
+## Response
+
+### Success (200)
+
+```json
+{
+  "barcode": "8000500310427",
+  "name": "Nutella",
+  "brand": "Ferrero",
+  "quantity": "750g",
+  "image_url": "https://...",
+  "category": "spreads",
+  "cached": false
+}
+```
+
+### Not Found (404)
+
+```json
+{
+  "barcode": "1234567890123",
+  "name": "Unknown Product (1234567890123)",
+  "cached": false
+}
+```
+
+### Error (500)
+
+```json
+{
+  "error": "Error message",
+  "barcode": null,
+  "name": null
+}
+```
+
+## Features
+
+- ✅ Queries Open Food Facts API
+- ✅ Caches results in `products` table
+- ✅ Returns cached data for subsequent requests
+- ✅ Handles product not found gracefully
+- ✅ CORS enabled for frontend access
+
+## Environment Variables
+
+- `SUPABASE_URL`: Auto-injected by Supabase
+- `SUPABASE_SERVICE_ROLE_KEY`: Auto-injected by Supabase
+
+## Testing
+
+```bash
+# Local (with Supabase CLI)
+supabase functions serve product-lookup
+
+# Test request
+curl -X POST http://localhost:54321/functions/v1/product-lookup \
+  -H "Content-Type: application/json" \
+  -H "Authorization: Bearer YOUR_ANON_KEY" \
+  -d '{"barcode":"8000500310427"}'
+```
+
+## Deployment
+
+```bash
+supabase functions deploy product-lookup
+```