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/frontend/src/routes/login/+page.svelte

@@ -12,6 +12,7 @@
 	let mode = $state<'login' | 'register'>('login');
 	let name = $state('');
 	let password = $state('');
+	let email = $state('');
 	let familyCode = $state('');
 	let error = $state('');
 	let loading = $state(false);
@@ -29,6 +30,7 @@
 		inviteLoading = true;
 		try {
 			invite = await api<InviteLookup>(`/api/auth/invite/${encodeURIComponent(token)}`);
+			email = invite.email;
 		} catch {
 			// Surface a clear message rather than the generic API error — the
 			// recipient probably arrived from a stale or revoked email link.
@@ -49,9 +51,9 @@
 			if (mode === 'login') {
 				body = { name, password };
 			} else if (inviteToken) {
-				body = { name, password, inviteToken };
+				body = { name, password, email, inviteToken };
 			} else {
-				body = { name, password, familyCode };
+				body = { name, password, email, familyCode };
 			}
 
 			const res = await api<{ token: string }>(endpoint, {
@@ -111,16 +113,28 @@
 				/>
 			</div>
 
-			{#if mode === 'register' && !inviteToken}
+			{#if mode === 'register'}
 				<div>
 					<input
-						type="text"
-						bind:value={familyCode}
-						placeholder="Family code"
+						type="email"
+						bind:value={email}
+						placeholder="Email"
 						required
-						class="w-full rounded-lg border border-gray-300 px-4 py-3 text-lg focus:border-primary focus:ring-2 focus:ring-primary/20 focus:outline-none"
+						readonly={!!inviteToken}
+						class="w-full rounded-lg border border-gray-300 px-4 py-3 text-lg focus:border-primary focus:ring-2 focus:ring-primary/20 focus:outline-none {inviteToken ? 'bg-gray-100' : ''}"
 					/>
 				</div>
+				{#if !inviteToken}
+					<div>
+						<input
+							type="text"
+							bind:value={familyCode}
+							placeholder="Family code"
+							required
+							class="w-full rounded-lg border border-gray-300 px-4 py-3 text-lg focus:border-primary focus:ring-2 focus:ring-primary/20 focus:outline-none"
+						/>
+					</div>
+				{/if}
 			{/if}
 
 			{#if error}