fix: Corriger la disparition des créneaux lors d'un déplacement inter-jour

Lors d'un drag & drop d'un créneau vers un autre jour de la semaine,
le nouveau créneau devenait invisible car recurrenceStart était fixé à
la date d'occurrence (le jour source). Si le jour cible tombait avant
cette date dans la semaine, isActiveOnDate retournait false.

recurrenceStart est maintenant fixé au lundi de la semaine d'occurrence,
ce qui garantit la visibilité du créneau dès la semaine du déplacement.

backend/tests/Unit/Scolarite/Application/Command/UpdateRecurringSlot/UpdateRecurringSlotHandlerTest.php

@@ -119,6 +119,41 @@ final class UpdateRecurringSlotHandlerTest extends TestCase
         self::assertTrue($result['newSlot']->teacherId->equals(UserId::fromString(self::NEW_TEACHER_ID)));
     }
 
+    #[Test]
+    public function allFutureCrossDayMoveSetsRecurrenceStartToWeekMonday(): void
+    {
+        // Wednesday slot — moving it to Monday
+        $slot = $this->createAndSaveSlot(
+            dayOfWeek: DayOfWeek::WEDNESDAY,
+            recurrenceStart: new DateTimeImmutable('2026-09-01'),
+            recurrenceEnd: new DateTimeImmutable('2027-07-04'),
+        );
+
+        $command = new UpdateRecurringSlotCommand(
+            tenantId: self::TENANT_ID,
+            slotId: (string) $slot->id,
+            occurrenceDate: '2026-10-14', // Wednesday
+            scope: 'all_future',
+            classId: self::CLASS_ID,
+            subjectId: self::SUBJECT_ID,
+            teacherId: self::TEACHER_ID,
+            dayOfWeek: DayOfWeek::MONDAY->value,
+            startTime: '08:00',
+            endTime: '09:00',
+            room: null,
+            updatedBy: self::UPDATER_ID,
+        );
+
+        $result = ($this->handler)($command);
+
+        $newSlot = $result['newSlot'];
+        self::assertNotNull($newSlot);
+        self::assertSame(DayOfWeek::MONDAY, $newSlot->dayOfWeek);
+        // recurrenceStart must be the Monday of the occurrence week (2026-10-12),
+        // not the occurrence date (2026-10-14), so the slot is visible this week
+        self::assertSame('2026-10-12', $newSlot->recurrenceStart->format('Y-m-d'));
+    }
+
     #[Test]
     public function allFutureWithNoOriginalEndUsesNullForNewSlotEnd(): void
     {
@@ -149,6 +184,7 @@ final class UpdateRecurringSlotHandlerTest extends TestCase
     }
 
     private function createAndSaveSlot(
+        DayOfWeek $dayOfWeek = DayOfWeek::MONDAY,
         ?DateTimeImmutable $recurrenceStart = null,
         ?DateTimeImmutable $recurrenceEnd = null,
     ): ScheduleSlot {
@@ -157,7 +193,7 @@ final class UpdateRecurringSlotHandlerTest extends TestCase
             classId: ClassId::fromString(self::CLASS_ID),
             subjectId: SubjectId::fromString(self::SUBJECT_ID),
             teacherId: UserId::fromString(self::TEACHER_ID),
-            dayOfWeek: DayOfWeek::MONDAY,
+            dayOfWeek: $dayOfWeek,
             timeSlot: new TimeSlot('08:00', '09:00'),
             room: null,
             isRecurring: true,