feat: Affectation des enseignants aux classes et matières

Permet aux administrateurs d'associer un enseignant à une classe pour une
matière donnée au sein d'une année scolaire. Cette brique est nécessaire
pour construire les emplois du temps et les carnets de notes par la suite.

Le modèle impose l'unicité du triplet enseignant × classe × matière par
année scolaire, avec réactivation automatique d'une affectation retirée
plutôt que duplication. L'isolation multi-tenant est garantie au niveau
du repository (findById/get filtrent par tenant_id).

backend/tests/Unit/Administration/Application/Command/RemoveAssignment/RemoveAssignmentHandlerTest.php

@@ -0,0 +1,121 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Tests\Unit\Administration\Application\Command\RemoveAssignment;
+
+use App\Administration\Application\Command\RemoveAssignment\RemoveAssignmentCommand;
+use App\Administration\Application\Command\RemoveAssignment\RemoveAssignmentHandler;
+use App\Administration\Domain\Exception\AffectationNotFoundException;
+use App\Administration\Domain\Model\SchoolClass\AcademicYearId;
+use App\Administration\Domain\Model\SchoolClass\ClassId;
+use App\Administration\Domain\Model\Subject\SubjectId;
+use App\Administration\Domain\Model\TeacherAssignment\AssignmentStatus;
+use App\Administration\Domain\Model\TeacherAssignment\TeacherAssignment;
+use App\Administration\Domain\Model\User\UserId;
+use App\Administration\Infrastructure\Persistence\InMemory\InMemoryTeacherAssignmentRepository;
+use App\Shared\Domain\Clock;
+use App\Shared\Domain\Tenant\TenantId;
+use DateTimeImmutable;
+use PHPUnit\Framework\Attributes\Test;
+use PHPUnit\Framework\TestCase;
+
+final class RemoveAssignmentHandlerTest extends TestCase
+{
+    private const string TENANT_ID = '550e8400-e29b-41d4-a716-446655440001';
+
+    private InMemoryTeacherAssignmentRepository $assignmentRepository;
+    private Clock $clock;
+
+    protected function setUp(): void
+    {
+        $this->assignmentRepository = new InMemoryTeacherAssignmentRepository();
+        $this->clock = new class implements Clock {
+            public function now(): DateTimeImmutable
+            {
+                return new DateTimeImmutable('2026-03-01 10:00:00');
+            }
+        };
+    }
+
+    #[Test]
+    public function itRemovesAssignmentSuccessfully(): void
+    {
+        $assignment = $this->createAndSaveAssignment();
+        $handler = new RemoveAssignmentHandler($this->assignmentRepository, $this->clock);
+
+        $command = new RemoveAssignmentCommand(
+            assignmentId: (string) $assignment->id,
+            tenantId: self::TENANT_ID,
+        );
+
+        $result = $handler($command);
+
+        self::assertSame(AssignmentStatus::REMOVED, $result->status);
+        self::assertFalse($result->estActive());
+        self::assertNotNull($result->endDate);
+    }
+
+    #[Test]
+    public function itThrowsExceptionWhenAssignmentNotFound(): void
+    {
+        $handler = new RemoveAssignmentHandler($this->assignmentRepository, $this->clock);
+
+        $command = new RemoveAssignmentCommand(
+            assignmentId: '550e8400-e29b-41d4-a716-446655440099',
+            tenantId: self::TENANT_ID,
+        );
+
+        $this->expectException(AffectationNotFoundException::class);
+        $handler($command);
+    }
+
+    #[Test]
+    public function itThrowsExceptionWhenTenantMismatch(): void
+    {
+        $assignment = $this->createAndSaveAssignment();
+        $handler = new RemoveAssignmentHandler($this->assignmentRepository, $this->clock);
+
+        $command = new RemoveAssignmentCommand(
+            assignmentId: (string) $assignment->id,
+            tenantId: '550e8400-e29b-41d4-a716-446655440099',
+        );
+
+        $this->expectException(AffectationNotFoundException::class);
+        $handler($command);
+    }
+
+    #[Test]
+    public function itIsIdempotentWhenAlreadyRemoved(): void
+    {
+        $assignment = $this->createAndSaveAssignment();
+        $handler = new RemoveAssignmentHandler($this->assignmentRepository, $this->clock);
+
+        $command = new RemoveAssignmentCommand(
+            assignmentId: (string) $assignment->id,
+            tenantId: self::TENANT_ID,
+        );
+
+        $result1 = $handler($command);
+        $result2 = $handler($command);
+
+        self::assertSame(AssignmentStatus::REMOVED, $result2->status);
+        self::assertEquals($result1->endDate, $result2->endDate);
+    }
+
+    private function createAndSaveAssignment(): TeacherAssignment
+    {
+        $assignment = TeacherAssignment::creer(
+            tenantId: TenantId::fromString(self::TENANT_ID),
+            teacherId: UserId::fromString('550e8400-e29b-41d4-a716-446655440010'),
+            classId: ClassId::fromString('550e8400-e29b-41d4-a716-446655440020'),
+            subjectId: SubjectId::fromString('550e8400-e29b-41d4-a716-446655440030'),
+            academicYearId: AcademicYearId::fromString('550e8400-e29b-41d4-a716-446655440040'),
+            createdAt: new DateTimeImmutable('2026-02-12 10:00:00'),
+        );
+
+        $this->assignmentRepository->save($assignment);
+
+        return $assignment;
+    }
+}