chore: remove Open Food Facts (OFF) integration

- Remove off-service.js and off-service.test.js
- Remove OFF import from server.js
- Simplify /api/products/search to only use CIMA
- Remove 'openfoodfacts' from valid product sources
- Update frontend-mobile Product interface to remove OFF source
- Remove OFF env vars from .env.example
- Add PARAPHARMACY_API_URL to .env.example

OFF was replaced by the new parapharmacy-api microservice.

apps/backend/.env.example

@@ -20,9 +20,5 @@ VAPID_SUBJECT=mailto:admin@example.com
 #   https://expo.dev/accounts/[username]/settings/access-tokens
 EXPO_ACCESS_TOKEN=
 
-# Open Food Facts
-# Register at: https://world.openfoodfacts.org/
-# User-Agent is REQUIRED per OFF docs (format: AppName/Version (ContactEmail))
-OFF_USER_AGENT=FarmaFinder/1.0 (https://github.com/farmafinder)
-OFF_USERNAME=
-OFF_PASSWORD=
+# Parapharmacy API
+PARAPHARMACY_API_URL=http://localhost:3002