fix(demo): scope category/item IDs per café to prevent PK collisions

DemoMenuSeeder used hardcoded IDs like cat_demo_coffee for every café.
If the dev seeder (runs when ASPNETCORE_ENVIRONMENT=Development) already
inserted those IDs for cafe_demo_001, a production café clicking
"Add demo data" hit a primary-key constraint violation.

Fix: EnsureMenuAsync now accepts useScopedIds=true which prefixes every
category and item ID with cafeId (e.g. cafe_abc_cat_demo_coffee).
CategoryId FKs on items are remapped through the same function.

DemoSeedService (the API endpoint handler) always passes useScopedIds=true.
DevelopmentDataSeeder keeps useScopedIds=false (default) so the existing
cafe_demo_001 rows in dev databases are not touched.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/Meezi.API/Services/DemoSeedService.cs

@@ -56,10 +56,12 @@ public class DemoSeedService : IDemoSeedService
                 .FirstAsync(ct);
         }
 
-        // 2. Seed menu (categories + items) using café-agnostic seeder
+        // 2. Seed menu (categories + items) using café-agnostic seeder.
+        // useScopedIds=true prefixes all IDs with cafeId so multiple cafés
+        // can each have their own demo menu without primary-key collisions.
         var beforeCats = await _db.MenuCategories.CountAsync(c => c.CafeId == cafeId, ct);
         var beforeItems = await _db.MenuItems.CountAsync(i => i.CafeId == cafeId, ct);
-        await DemoMenuSeeder.EnsureMenuAsync(_db, cafeId, taxId, _logger);
+        await DemoMenuSeeder.EnsureMenuAsync(_db, cafeId, taxId, _logger, useScopedIds: true);
         var afterCats = await _db.MenuCategories.CountAsync(c => c.CafeId == cafeId, ct);
         var afterItems = await _db.MenuItems.CountAsync(i => i.CafeId == cafeId, ct);