fix(seed): IgnoreQueryFilters on all seeder queries + sitemap invalid date guard

DemoSeedService / DemoMenuSeeder:
  Add IgnoreQueryFilters() to every seeder lookup (Taxes, MenuCategories,
  MenuItems). Soft-deleted rows still hold their PKs; without this a second
  seed run after user-deletion throws a PK collision on the Tax or category
  that was soft-deleted but is still in the index.

sitemap.ts:
  Guard new Date(post.date) against empty / missing frontmatter date fields.
  new Date("") = Invalid Date → broken <lastmod> in sitemap XML.
  Fall back to the build-time date when the post date is absent or invalid.

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

src/Meezi.API/Services/DemoSeedService.cs

@@ -33,7 +33,9 @@ public class DemoSeedService : IDemoSeedService
         // 1. Ensure 9% default tax
         var taxId = $"{cafeId}_demo_tax";
         var taxCreated = false;
-        if (!await _db.Taxes.AnyAsync(t => t.CafeId == cafeId && t.IsDefault, ct))
+        // IgnoreQueryFilters: soft-deleted rows still occupy the PK; re-seeding
+        // after a user deletes demo data must see those rows to avoid a PK collision.
+        if (!await _db.Taxes.IgnoreQueryFilters().AnyAsync(t => t.CafeId == cafeId && t.IsDefault, ct))
         {
             _db.Taxes.Add(new Tax
             {
@@ -51,6 +53,7 @@ public class DemoSeedService : IDemoSeedService
         else
         {
             taxId = await _db.Taxes
+                .IgnoreQueryFilters()
                 .Where(t => t.CafeId == cafeId && t.IsDefault)
                 .Select(t => t.Id)
                 .FirstAsync(ct);

src/Meezi.Infrastructure/Data/DemoMenuSeeder.cs

@@ -23,7 +23,7 @@ public static class DemoMenuSeeder
         string Scoped(string catalogId) =>
             useScopedIds ? $"{cafeId}_{catalogId}" : catalogId;
 
-        if (!await db.Taxes.AnyAsync(t => t.Id == taxId && t.CafeId == cafeId))
+        if (!await db.Taxes.IgnoreQueryFilters().AnyAsync(t => t.Id == taxId && t.CafeId == cafeId))
         {
             db.Taxes.Add(new Tax
             {
@@ -38,6 +38,7 @@ public static class DemoMenuSeeder
         }
 
         var existingCategoryIds = await db.MenuCategories
+            .IgnoreQueryFilters()
             .Where(c => c.CafeId == cafeId)
             .ToDictionaryAsync(c => c.Id, StringComparer.Ordinal);
 
@@ -78,6 +79,7 @@ public static class DemoMenuSeeder
         }
 
         var existingItemIds = await db.MenuItems
+            .IgnoreQueryFilters()
             .Where(i => i.CafeId == cafeId)
             .Select(i => i.Id)
             .ToListAsync();