Add invite-only group classes

Group classes can now be marked invite-only (us_offerings.access_mode).
Invite-only classes are hidden from the public catalog and reachable only
when the instructor lets someone in via one of three paths, managed from
My Lessons -> My Group Classes:

- Add students directly: enrols them now with a pending payment.
- Make available: grants registered students access to self-enrol through
  the normal paid flow (multi-select, emailed a notice).
- Invite by email: tokenised registration invite tied to the class for a
  non-account address; after they register the class becomes enrollable.
  Reuses an existing pending invite instead of sending a second link.

New us_group_access table records grants; GET /offerings merges granted
invite-only classes for the caller; enrolment requires a grant
(403 invite_required) and flips it to enrolled on success.

composer test (487), composer lint, composer cs all pass.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

tests/Unit/Offering/OfferingControllerTest.php

@@ -70,6 +70,39 @@ class OfferingControllerTest extends TestCase
         $this->render();
     }
 
+    public function testAddInviteOnlyGroupClassStoresInviteOnlyAccess(): void
+    {
+        $_POST = [
+            'usc_action'  => 'add',
+            'title'       => 'Private Choir',
+            'kind'        => Offering::KIND_GROUP_CLASS,
+            'invite_only' => '1',
+        ];
+
+        $this->repository->shouldReceive('insert')->once()->with(Mockery::on(
+            static fn (Offering $o) => Offering::ACCESS_INVITE_ONLY === $o->accessMode
+        ))->andReturn(1);
+        $this->repository->shouldReceive('findAll')->andReturn([]);
+
+        $this->render();
+    }
+
+    public function testAddWithoutInviteOnlyDefaultsToPublicAccess(): void
+    {
+        $_POST = [
+            'usc_action' => 'add',
+            'title'      => 'Open Choir',
+            'kind'       => Offering::KIND_GROUP_CLASS,
+        ];
+
+        $this->repository->shouldReceive('insert')->once()->with(Mockery::on(
+            static fn (Offering $o) => Offering::ACCESS_PUBLIC === $o->accessMode
+        ))->andReturn(1);
+        $this->repository->shouldReceive('findAll')->andReturn([]);
+
+        $this->render();
+    }
+
     public function testAddOneOffGroupClassEndsOnItsStartDate(): void
     {
         $_POST = [