Collapse migrations, require email at registration

No deployed environments yet, so consolidate the entire migration history
into a single Initial migration and tighten the schema accordingly.

- User.Email is now non-nullable (required); the partial unique index
  used to tolerate legacy null emails is gone in favor of a plain
  unique index.
- Both register paths require an email up front. Invite-path
  registrations must match the invited address (server ignores any
  mismatched client value); family-code registrations bind whatever the
  user supplies but stay unconfirmed (EmailConfirmedAt = null) since
  the family code does not prove email ownership.
- Validation order in /register reworked: invite/family-code resolution
  runs before duplicate-name/email checks so a consumed token surfaces
  a clean "invitation invalid" error instead of getting masked by the
  duplicate-email response.
- All 14 prior migrations replaced with a single Initial migration.
- Test fixtures, builders, and unit tests updated to supply emails.
- Login page register form now collects an email field; invite-bound
  registrations show the invited address as a read-only input.

Local dev DBs need to be recreated (drop the yeschef-pgdata volume or
the yeschef Postgres database). No production data exists yet.

src/backend/YesChef.Api.IntegrationTests/Builders/UserBuilder.cs

@@ -8,15 +8,22 @@ public sealed class UserBuilder
 {
     private string _name = $"user-{Guid.NewGuid():N}"[..16];
     private string _password = "correct-horse-battery-staple";
+    private string? _email;
 
     public UserBuilder Named(string name) { _name = name; return this; }
     public UserBuilder WithPassword(string password) { _password = password; return this; }
+    public UserBuilder WithEmail(string email) { _email = email; return this; }
 
     public string PlaintextPassword => _password;
 
     public User Build()
     {
-        var user = new User { Name = _name, PasswordHash = "" };
+        var user = new User
+        {
+            Name = _name,
+            PasswordHash = "",
+            Email = _email ?? $"{_name}@example.test",
+        };
         user.PasswordHash = new PasswordHasher<User>().HashPassword(user, _password);
         return user;
     }