Enable offline product search functionality

Implement offline search using the [Open Food Facts data export](https://world.openfoodfacts.org/data).

The issue mentions using the Open Food Facts data export to implement offline search. First, examine the existing API endpoints in the FastAPI server to understand how product data is currently handled. Look for any existing search functionality or data models. Determine where and how the offline data export would be stored and indexed (e.g., local SQLite, Elasticsearch). The 'done' state is a new endpoint or enhanced existing endpoint that can search locally without an internet connection.

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