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/InviteTest.php

@@ -95,6 +95,52 @@ class InviteTest extends TestCase
         self::assertFalse($invite->isAcceptable('2026-06-02 09:00:00'));
     }
 
+    public function testGroupInviteHonoursExplicitExpiry(): void
+    {
+        $invite = new Invite(
+            '',
+            'tok',
+            createdAt: '2026-06-01 09:00:00',
+            kind: Invite::KIND_GROUP,
+            expiresAt: '2026-08-31 23:59:59'
+        );
+
+        self::assertTrue($invite->isGroup());
+        // 19 days after creation — past the personal 14-day window, but the
+        // explicit expiry governs.
+        self::assertFalse($invite->isExpired('2026-06-20 09:00:00'));
+        self::assertTrue($invite->isAcceptable('2026-06-20 09:00:00'));
+
+        self::assertTrue($invite->isExpired('2026-09-01 00:00:00'));
+        self::assertFalse($invite->isAcceptable('2026-09-01 00:00:00'));
+    }
+
+    public function testExplicitExpiryWinsOverCreationWindowForPersonalInvites(): void
+    {
+        $invite = new Invite('[email protected]', 'tok', createdAt: '2026-06-01 09:00:00', expiresAt: '2026-06-02 23:59:59');
+
+        // One day old (inside the 14-day window) but past its explicit expiry.
+        self::assertTrue($invite->isExpired('2026-06-03 09:00:00'));
+    }
+
+    public function testFromRowDefaultsKindWhenColumnMissing(): void
+    {
+        $invite = Invite::fromRow((object) [
+            'id'               => '5',
+            'email'            => '[email protected]',
+            'token'            => 'tok123',
+            'role'             => RoleManager::STUDENT,
+            'status'           => Invite::STATUS_PENDING,
+            'invited_by'       => null,
+            'accepted_user_id' => null,
+            'accepted_at'      => null,
+        ]);
+
+        self::assertSame(Invite::KIND_PERSONAL, $invite->kind);
+        self::assertFalse($invite->isGroup());
+        self::assertNull($invite->expiresAt);
+    }
+
     public function testHashTokenIsDeterministicSha256(): void
     {
         $hash = Invite::hashToken('raw-token');
@@ -109,7 +155,7 @@ class InviteTest extends TestCase
     {
         $arr = (new Invite('[email protected]', 'tok', id: 1))->toArray();
 
-        foreach (['id', 'email', 'token', 'role', 'status', 'invited_by', 'accepted_user_id', 'accepted_at'] as $key) {
+        foreach (['id', 'email', 'token', 'role', 'kind', 'status', 'invited_by', 'accepted_user_id', 'accepted_at', 'expires_at'] as $key) {
             self::assertArrayHasKey($key, $arr);
         }
     }