Bug: Product edits don't fully save

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Phase 2 — Bug fix (required).

Admins report that when they edit a product, some changes don't stick. For example, updating
a product's stock returns OK but the value in the database never actually changes
(other fields such as price and description update correctly).

Tasks

• Reproduce by editing a product's stock via PUT /products/{id} and re-reading it
• Find and fix the cause so every field updates
• Add a unit test that proves the fix

Done when

• Editing any field (including stock) persists correctly