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/InviteRepositoryTest.php

@@ -34,16 +34,40 @@ class InviteRepositoryTest extends TestCase
                 Mockery::on(static function (array $d): bool {
                     return $d['email'] === '[email protected]'
                         && $d['token'] === 'tok123'
+                        && $d['kind'] === Invite::KIND_PERSONAL
                         && $d['status'] === Invite::STATUS_PENDING
-                        && $d['invited_by'] === 2;
+                        && $d['invited_by'] === 2
+                        && $d['expires_at'] === null;
                 }),
-                ['%s', '%s', '%s', '%s', '%d', '%d', '%s', '%s']
+                ['%s', '%s', '%s', '%s', '%s', '%d', '%d', '%s', '%s', '%s']
             );
         $this->db->insert_id = 5;
 
         self::assertSame(5, $this->repo->insert(new Invite('[email protected]', 'tok123', invitedBy: 2)));
     }
 
+    public function testInsertPersistsGroupKindAndExpiry(): void
+    {
+        Functions\expect('current_time')->with('mysql')->andReturn('2026-06-02 09:00:00');
+
+        $this->db->shouldReceive('insert')
+            ->once()
+            ->with(
+                'wp_us_invites',
+                Mockery::on(static function (array $d): bool {
+                    return $d['email'] === ''
+                        && $d['kind'] === Invite::KIND_GROUP
+                        && $d['expires_at'] === '2026-08-31 23:59:59';
+                }),
+                Mockery::type('array')
+            );
+        $this->db->insert_id = 6;
+
+        $invite = new Invite('', 'tok456', invitedBy: 2, kind: Invite::KIND_GROUP, expiresAt: '2026-08-31 23:59:59');
+
+        self::assertSame(6, $this->repo->insert($invite));
+    }
+
     public function testFindByTokenReturnsInvite(): void
     {
         $this->db->shouldReceive('prepare')