Add product catalog with per-store section memory

Introduces a global Products catalog plus per-family overrides and
private FamilyProducts, exposed via /api/products with a merged
search. Shopping list items and recipe ingredients gain optional
ProductId/FamilyProductId links, and a new ProductStoreSection
table remembers which section a product was last placed in at a
given store so future adds auto-assign the right section.

Frontend gets a reusable ProductTypeahead component, wired into
list-item add and recipe ingredient entry with free-form fallback.

A startup CatalogSeeder loads ~115 curated staples from an embedded
JSON resource via INSERT ... ON CONFLICT DO NOTHING; skipped under
the Testing environment so integration tests keep a clean slate.

src/backend/YesChef.Api/Entities/FamilyProductOverride.cs

@@ -0,0 +1,19 @@
+namespace YesChef.Api.Entities;
+
+/// <summary>
+/// A family's override of a global <see cref="Product"/>. Each non-null field
+/// replaces the global value when the family views the catalog. Composite key
+/// (<see cref="FamilyId"/>, <see cref="ProductId"/>) — at most one override
+/// per family per product.
+/// </summary>
+public class FamilyProductOverride
+{
+    public int FamilyId { get; set; }
+    public Family Family { get; set; } = null!;
+    public int ProductId { get; set; }
+    public Product Product { get; set; } = null!;
+    public string? Name { get; set; }
+    public string? Brand { get; set; }
+    public string? Notes { get; set; }
+    public DateTime UpdatedAt { get; set; } = DateTime.UtcNow;
+}