Add Barcode Lookup API driver

[jpdevries]

Summary

Add a Barcode Lookup driver for the pluggable barcode system (#13).

Why

Barcode Lookup covers 500M+ barcodes across food and non-food products — cleaning supplies, batteries, pet food, etc. Useful for households that want to scan everything, not just groceries.

Notes

• Paid API ($10+/month), so this is opt-in via BARCODE_DRIVER=barcodelookup
• Requires BARCODE_LOOKUP_API_KEY env var
• Default remains Open Food Facts (free)

Depends on #13 (driver pattern).

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