Add multi-use group invite links with expiry and auto-approval on email confirmation

A studio admin can generate a shareable group invite link (e.g. for a
newsletter) from the Invites page, choosing a required expiry date. Anyone
with the link may register while it is valid, in any registration mode: the
form collects their own email, they must confirm it via the usual hashed
token, and confirming approves the account immediately — group signups never
enter the Pending Students queue.

- us_invites grows kind (personal/group) and expires_at; an explicit expiry
  wins over the personal 14-day window. Group links stay pending (multi-use)
  until revoked or expired.
- RegistrationPage: group signups create the account pending with the
  us_auto_approve marker and send the confirmation email; no auto-login.
- EmailConfirmationHandler: auto-approve accounts are approved on
  confirmation, emailed the approved notice, and redirected to a new
  us_confirmed=ready notice with a sign-in link.

Closes #77

Co-Authored-By: Claude Fable 5 <[email protected]>

tests/Unit/Auth/RegistrationPageTest.php

@@ -123,6 +123,79 @@ class RegistrationPageTest extends TestCase
         self::assertSame('confirm', $this->submit(null, true));
     }
 
+    public function testGroupInviteCreatesPendingAutoApproveAccountEvenWhenClosed(): void
+    {
+        $_POST = [ 'password' => 'password123', 'display_name' => 'Ada', 'email' => '[email protected]' ];
+
+        Functions\when('is_email')->justReturn(true);
+        Functions\when('email_exists')->justReturn(false);
+        Functions\when('wp_insert_user')->justReturn(42);
+        Functions\when('is_wp_error')->justReturn(false);
+        Functions\when('wp_generate_password')->justReturn('rawtok');
+        // confirmUrl internals
+        Functions\when('get_option')->justReturn(0);
+        Functions\when('home_url')->justReturn('http://home.test/');
+        Functions\when('add_query_arg')->alias(static fn (string $k, string $v, string $u): string => $u . '?' . $k . '=' . $v);
+
+        // The auto-approve marker must be set alongside the pending metas.
+        $metas = [];
+        Functions\when('update_user_meta')->alias(static function (int $id, string $key, $value) use (&$metas): bool {
+            $metas[$key] = $value;
+            return true;
+        });
+
+        $user = Mockery::mock(\WP_User::class);
+        Functions\when('get_user_by')->justReturn($user);
+
+        $this->ctx['mailer']->shouldReceive('sendConfirmation')->once()->with($user, Mockery::type('string'));
+        // The link is multi-use: never marked accepted, and no auto-login.
+        $this->ctx['invites']->shouldReceive('markAccepted')->never();
+        Functions\expect('wp_set_auth_cookie')->never();
+
+        $invite = new Invite(
+            email: '',
+            token: 'hash',
+            createdAt: '2024-01-01 00:00:00',
+            kind: Invite::KIND_GROUP,
+            expiresAt: '2024-02-01 23:59:59',
+            id: 9
+        );
+
+        // Registration mode is invite-only (open = false): the group link still works.
+        self::assertSame('confirm_group', $this->submit($invite, false));
+        self::assertSame('1', $metas['us_auto_approve'] ?? null);
+    }
+
+    public function testGroupInviteRendersEditableEmailField(): void
+    {
+        $_REQUEST = ['us_invite' => 'raw-token'];
+        Functions\when('is_user_logged_in')->justReturn(false);
+        Functions\when('sanitize_key')->alias(static fn ($v) => strtolower((string) $v));
+        Functions\when('wp_login_url')->justReturn('http://home.test/wp-login.php');
+        Functions\when('wp_nonce_field')->justReturn('');
+        // Invite-only mode: only the group link grants access to the form.
+        $this->ctx['settings']->shouldReceive('openRegistrationEnabled')->andReturn(false);
+
+        $invite = new Invite(
+            email: '',
+            token: 'hash',
+            createdAt: '2024-01-01 00:00:00',
+            kind: Invite::KIND_GROUP,
+            expiresAt: '2024-02-01 23:59:59',
+            id: 9
+        );
+        $this->ctx['invites']->shouldReceive('findByToken')
+            ->once()
+            ->with(Invite::hashToken('raw-token'))
+            ->andReturn($invite);
+
+        $html = $this->ctx['page']->render([]);
+
+        self::assertStringContainsString('<form', $html);
+        self::assertStringContainsString('name="email"', $html);
+        self::assertStringNotContainsString('readonly', $html);
+    }
+
     public function testClosedModeWithoutInviteReturnsError(): void
     {
         $result = $this->submit(null, false);