Pre-fill list section on product pick; tighten backend warnings

- Adds GET /api/products/{kind}/{id}/section?storeId=... exposing the
  per-store memory the list page mirrors when a product is picked, so the
  section dropdown reflects what the backend would auto-assign on POST.
- Treats backend warnings as errors via Directory.Build.props; fixes the
  surfaced warnings (obsolete PostgreSqlBuilder ctor, nullable string[]
  in IsEquivalentTo, redundant nullable flow).
- Annotates wire-exposed enums (ProductKind, UnitKind, UnitCategory,
  UnitCategoryFlags) with JsonStringEnumConverter so they round-trip as
  strings regardless of caller options. Unblocks the integration tests
  that deserialize DTOs via GetFromJsonAsync without the global converter.

BACKLOG.md

@@ -55,10 +55,10 @@ The product-catalog foundation has shipped. A `Product` is the canonical thing b
 - **Catalog management page (`/products`):** search the effective catalog, add a family product, edit a global product (writes an override) or family product, "Reset to catalog default" for an overridden global, and delete a family product. Linked from the lists page.
 - **Override indicator:** "Edited" pill on overridden rows in both the catalog page and the typeahead dropdown.
 - **`ProductStoreSection` write path:** when an item is saved/checked with `(productId, sectionId)`, the mapping is remembered for `(family, store, product)`.
+- **Auto-assign section from product on add.** `GET /api/products/{kind}/{id}/section?storeId=...` exposes the remembered mapping; `onItemProductChange` on the list page calls it and pre-fills the section dropdown so the user can see (and override) what the backend would auto-assign. The recipe → list copy path already lands ingredients in remembered sections server-side.
 - **Seed data:** ~50 hand-curated common-groceries items in `Data/Seed/products.json`, applied by `CatalogSeeder` on startup (idempotent on `Name`).
 
 #### Remaining
-- **Auto-assign section from product on add.** The write path remembers `(product, store) → section` and the backend has a helper that reads it back, but the add-item form doesn't call it on product pick yet. Described in detail in *Auto-assign section from product* further down.
 - **"Add '<text>' as a new product" affordance in the typeahead.** Today the typeahead only suggests existing catalog rows; an unmatched name becomes a pure-text item. Adding an explicit affordance to promote that name into a `FamilyProduct` from the same dropdown is still open.
 - **Seed expansion to ~2–3k curated items.** Current seed file has ~50 entries. Growth is a data exercise, not a code one — keep `products.json` the source of truth, keep the seeder idempotent on `Name`.
 - **Catalog ingestion tooling (future).** When the curated list starts feeling limiting, build re-runnable importers for public datasets so we don't grow by hand-typing.
@@ -133,22 +133,9 @@ Replace free-form `Quantity` strings with a structured `(Quantity, UnitOfMeasure
 - "A few", "to taste", "some" — these are real recipe quantities that don't fit `(decimal, unit)`. Probably modeled as a special `IsApproximate` flag with an optional `QuantityNote` rather than forcing them into the structured shape.
 
 ### Per-store sections — remaining polish
-The base feature is shipped (entity, default seed on store create, list view groups by section). Remaining nice-to-haves:
-- **Per-store ingredient memory:** remember "last time `Bananas` was bought at Kroger it was in Produce" and auto-assign on next add at that store. Adds an `IngredientSection` mapping table per store. Pairs naturally with the product catalog.
-- **Recipes → sections:** when pulling recipe ingredients into a list, map them to the list's store's sections (only meaningful once the per-store ingredient memory or product catalog lands).
+The base feature is shipped (entity, default seed on store create, list view groups by section, per-store ingredient memory via `ProductStoreSection` auto-assigning on item add and recipe-to-list copy). Remaining nice-to-haves:
 - **Section drag-to-reorder** in the store edit UI — section walk order matters, but reordering today only works by editing `SortOrder` numbers manually.
 
-### Auto-assign section from product
-When a user picks a product from the typeahead on the shopping list add form, pre-populate the section dropdown with that product's known section for the current store — rather than leaving it as "Uncategorized".
-
-This is the "per-store ingredient memory" item above, stated from the user's perspective: choosing "Spaghetti" should already know it belongs in Pasta/Dry Goods at this store.
-
-**State of play:** `ProductStoreSection` is shipped — table, indexes, and the write path that records `(family, store, product) → section` whenever an item with a product link is saved with a chosen section. A backend helper in `ShoppingListEndpoints` already reads the effective section back. **The missing piece is the frontend:** `onItemProductChange` in `lists/[id]/+page.svelte` doesn't call the read path on product pick yet, so the section dropdown still defaults to "Uncategorized" until the user sets it.
-
-**To finish:** expose the lookup via an endpoint (`GET /api/products/{id}/section?storeId=...` or roll it into the typeahead response when a `storeId` is supplied), call it on `onItemProductChange`, and pre-select the returned section. Same treatment for the recipe → list copy path so adding a recipe to a list lands ingredients in the right sections.
-
-**Scope note:** the section pre-fill is family-scoped memory (`ProductStoreSection` rows they've created), not a global default — different families organize differently.
-
 ## Recipes
 
 ### Structured multi-step instructions