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/Migrations/YesChefDbModelSnapshot.cs

@@ -414,6 +414,7 @@ namespace YesChef.Api.Migrations
                         .HasColumnType("timestamp with time zone");
 
                     b.Property<string>("Email")
+                        .IsRequired()
                         .HasMaxLength(254)
                         .HasColumnType("character varying(254)");
 
@@ -432,8 +433,7 @@ namespace YesChef.Api.Migrations
                     b.HasKey("Id");
 
                     b.HasIndex("Email")
-                        .IsUnique()
-                        .HasFilter("\"Email\" IS NOT NULL");
+                        .IsUnique();
 
                     b.HasIndex("Name")
                         .IsUnique();