Introduce Family entity and bootstrap default family on startup

Foundation for the multi-tenant migration: adds the Family table with a
unique InviteCode, and a startup hook that bootstraps a single default
family from the FamilyCode config when the table is empty. No behavior
change yet — the table exists and is seeded but nothing reads it.

Also fixes the backend test command in CLAUDE.md: dotnet test on the
.NET 10 SDK with MTP rejects the --solution switch and positional
project args, so we now use Push-Location + --project.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/backend/YesChef.Api/Migrations/YesChefDbModelSnapshot.cs

@@ -1,4 +1,4 @@
-// <auto-generated />
+// <auto-generated />
 using System;
 using Microsoft.EntityFrameworkCore;
 using Microsoft.EntityFrameworkCore.Infrastructure;
@@ -22,6 +22,35 @@ namespace YesChef.Api.Migrations
 
             NpgsqlModelBuilderExtensions.UseIdentityByDefaultColumns(modelBuilder);
 
+            modelBuilder.Entity("YesChef.Api.Entities.Family", b =>
+                {
+                    b.Property<int>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("integer");
+
+                    NpgsqlPropertyBuilderExtensions.UseIdentityByDefaultColumn(b.Property<int>("Id"));
+
+                    b.Property<DateTime>("CreatedAt")
+                        .HasColumnType("timestamp with time zone");
+
+                    b.Property<string>("InviteCode")
+                        .IsRequired()
+                        .HasMaxLength(100)
+                        .HasColumnType("character varying(100)");
+
+                    b.Property<string>("Name")
+                        .IsRequired()
+                        .HasMaxLength(100)
+                        .HasColumnType("character varying(100)");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("InviteCode")
+                        .IsUnique();
+
+                    b.ToTable("Families");
+                });
+
             modelBuilder.Entity("YesChef.Api.Entities.Recipe", b =>
                 {
                     b.Property<int>("Id")