WEB-0: CMS content model + /api/site-snapshot + Page/Block CRUD

- Entities: Site (singleton, navOrder JSON), Page (slug/title/seoDescription/status enum),
  Block (single table, Type enum + Data JSON; RichText=Markdown), Media. Migration
  AddCmsEntities, seeds 6 Jimdo-mirror pages (placeholder Heading each; abgabetiere also an
  auto AbgabetiereList block) + Site singleton nav.
- GET /api/site-snapshot: one JSON doc (site nav + pages + ordered blocks {id,order,type,data});
  AbgabetiereList(auto) resolved from live ForSale gerbils -> {name, farbschlag, group(=Becken),
  photos[urls], aiSaleText:null} (no price per god). SiteSnapshotService.
- CRUD: GET/POST/PUT/DELETE pages; add/update/delete blocks; PUT .../blocks/order reorder.
  Block type allowlisted by enum; data validated as JSON object.

GerbilManagerWebAPI/ApplicationContext.cs

@@ -17,6 +17,10 @@ public class ApplicationContext : DbContext
     public DbSet<WeightRecord> WeightRecords => Set<WeightRecord>();
     public DbSet<SaleContract> SaleContracts => Set<SaleContract>();
     public DbSet<BreederSettings> BreederSettings => Set<BreederSettings>();
+    public DbSet<Site> Sites => Set<Site>();
+    public DbSet<Page> Pages => Set<Page>();
+    public DbSet<Block> Blocks => Set<Block>();
+    public DbSet<Media> Media => Set<Media>();
 
     // Keep Gerbil.NameSearch in sync on every save (separator-insensitive search key),
     // so it can never drift from Name regardless of which code path mutates the entity.
@@ -122,9 +126,85 @@ public class ApplicationContext : DbContext
         modelBuilder.Entity<BreederSettings>()
             .HasData(new BreederSettings { Id = GerbilManagerWebAPI.Models.BreederSettings.SingletonId });
 
+        // WEB epic: CMS content model.
+        modelBuilder.Entity<Site>(e =>
+        {
+            var navConverter = new Microsoft.EntityFrameworkCore.Storage.ValueConversion.ValueConverter<List<Guid>, string>(
+                v => System.Text.Json.JsonSerializer.Serialize(v, (System.Text.Json.JsonSerializerOptions?)null),
+                v => string.IsNullOrEmpty(v)
+                    ? new List<Guid>()
+                    : System.Text.Json.JsonSerializer.Deserialize<List<Guid>>(v, (System.Text.Json.JsonSerializerOptions?)null) ?? new List<Guid>());
+            var navComparer = new Microsoft.EntityFrameworkCore.ChangeTracking.ValueComparer<List<Guid>>(
+                (a, b) => (a ?? new List<Guid>()).SequenceEqual(b ?? new List<Guid>()),
+                v => v == null ? 0 : v.Aggregate(0, (h, x) => HashCode.Combine(h, x.GetHashCode())),
+                v => v.ToList());
+            e.Property(s => s.NavOrder).HasConversion(navConverter, navComparer);
+        });
+
+        modelBuilder.Entity<Page>(e =>
+        {
+            e.HasIndex(p => p.Slug).IsUnique();
+            e.Property(p => p.Status).HasConversion<string>();
+            e.HasMany(p => p.Blocks).WithOne()
+                .HasForeignKey(b => b.PageId).OnDelete(DeleteBehavior.Cascade);
+        });
+
+        modelBuilder.Entity<Block>(e => e.Property(b => b.Type).HasConversion<string>());
+
+        SeedCms(modelBuilder);
         SeedColorVarieties(modelBuilder);
     }
 
+    /// <summary>
+    /// Seed the public website's CMS: one Site + the 6 Jimdo-mirror pages, each with a
+    /// placeholder Heading; the abgabetiere page also carries an auto AbgabetiereList block
+    /// (resolved from live ForSale animals at snapshot time). Hand-editable afterwards.
+    /// </summary>
+    private static void SeedCms(ModelBuilder modelBuilder)
+    {
+        Guid Pid(int n) => new($"51720001-0000-0000-0000-{n:D12}");
+        Guid Bid(int n) => new($"51720002-0000-0000-0000-{n:D12}");
+
+        (int n, string Slug, string Title)[] pages =
+        {
+            (1, "start", "Startseite"),
+            (2, "ueber-die-zucht", "Über die Zucht"),
+            (3, "abgabetiere", "Abgabetiere"),
+            (4, "abgabebedingungen", "Abgabebedingungen"),
+            (5, "farben-genetik", "Farben & Genetik"),
+            (6, "kontakt", "Kontakt"),
+        };
+
+        var pageRows = new List<Page>();
+        var blockRows = new List<Block>();
+        foreach (var p in pages)
+        {
+            pageRows.Add(new Page { Id = Pid(p.n), Slug = p.Slug, Title = p.Title, Status = PageStatus.Published });
+            // placeholder Heading per page
+            blockRows.Add(new Block
+            {
+                Id = Bid(p.n), PageId = Pid(p.n), Order = 0, Type = BlockType.Heading,
+                Data = $"{{\"text\":\"{p.Title}\",\"level\":1}}",
+            });
+        }
+        // abgabetiere (n=3): auto AbgabetiereList as the second block
+        blockRows.Add(new Block
+        {
+            Id = Bid(10), PageId = Pid(3), Order = 1, Type = BlockType.AbgabetiereList,
+            Data = "{\"mode\":\"auto\",\"intro\":\"\"}",
+        });
+
+        modelBuilder.Entity<Page>().HasData(pageRows);
+        modelBuilder.Entity<Block>().HasData(blockRows);
+
+        modelBuilder.Entity<Site>().HasData(new Site
+        {
+            Id = Site.SingletonId,
+            DefaultLocale = "de",
+            NavOrder = pages.Select(p => Pid(p.n)).ToList(),
+        });
+    }
+
     /// <summary>
     /// Seed the colour-variety catalog. Source of truth = Kevin's GEN-2 generated list
     /// (gerbil-manager-web/src/genetics/colorVarietySeed.generated.json): 73 varieties