[code811]

Summary

Adds Shopify match indicators and per-product sync timestamps to the Products & Reorder Thresholds page while preserving existing Hub records.

• GET /products calculates matched or unmatched using Shopify inventory item ID or SKU.
• If Shopify retrieval fails, Hub products remain available with an “Unable to Verify” status.
• Adds nullable last_synced_at timestamps, updated when a product is successfully written through Sync from Shopify.
• Displays status pills, formatted timestamps, and “Never Recorded” where no sync timestamp exists.
• Includes a hover explanation distinguishing the Hub sync time from Shopify data freshness; synchronization may use cached data.

Validation performed

Local sample-mode runtime checks confirmed:

• Eight sample products matched; four Hub-only products remained unmatched and were retained.
• Inventory-ID matching worked with a different sample SKU.
• Successful and failed Shopify retrieval paths displayed the expected API values and UI states.
• The schema applied successfully, with existing timestamps initially null.
• Synchronization populated timestamps for processed products; subsequent synchronization advanced them.
• GET requests did not change stored timestamps.
• Timestamp persistence was checked through both the API and PostgreSQL.

Checks to record before merge

• Frontend build: npm --prefix frontend run build — result not yet recorded.
• Existing backend tests: npm --prefix backend test — result not yet recorded.

Remaining unverified

• Live Shopify comparison and resolution of the originally reported deployed product mismatch.
• Threshold associations against the correct live Shopify products.
• Timestamp behavior for newly inserted products and individual failed database writes.
• Deployment and schema application outside the local environment.

Product deactivation and changes to the identifier strategy are outside this PR’s scope.