feat(gestion): suppression en cascade exercice/programme/séance-modèle (ticket #37)

Ajoute la suppression avec gestion des dépendances en cascade sur les
écrans bibliothèque d'exercices, programme et séance-modèle, avec les
use cases et le bootstrap applicatif associés. flutter analyze propre,
58/58 tests verts, build APK debug validé.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

test/presentation/exercise_library_screen_test.dart

@@ -148,17 +148,87 @@ void main() {
       expect(find.text('Image importée.'), findsOneWidget);
     },
   );
+
+  testWidgets(
+    'supprimer un exercice demande confirmation puis le retire de la liste',
+    (tester) async {
+      final exerciseRepository = _FakeExerciseRepository()
+        ..referenced = true
+        ..exercises.add(
+          Exercise(
+            metadata: _metadata('exercise-1'),
+            name: 'Squat',
+            hasTimeMeasure: true,
+            hasRepsMeasure: false,
+            hasScoreMeasure: false,
+            defaultTargetTimeSeconds: 45,
+          ),
+        );
+
+      await tester.pumpWidget(
+        MaterialApp(
+          home: ExerciseLibraryScreen(
+            exerciseUseCases: _exerciseUseCases(exerciseRepository),
+            mediaUseCases: _mediaUseCases(exerciseRepository),
+          ),
+        ),
+      );
+      await tester.pump();
+      await tester.pump();
+
+      await tester.tap(find.byTooltip('Supprimer l’exercice'));
+      await tester.pumpAndSettle();
+
+      expect(find.text('Supprimer Squat ?'), findsOneWidget);
+      expect(
+        find.textContaining('Le supprimer le retirera de ces programmes.'),
+        findsOneWidget,
+      );
+
+      await tester.tap(find.text('Annuler'));
+      await tester.pumpAndSettle();
+
+      expect(find.text('Squat'), findsOneWidget);
+      expect(exerciseRepository.saved, isEmpty);
+
+      await tester.tap(find.byTooltip('Supprimer l’exercice'));
+      await tester.pumpAndSettle();
+      await tester.tap(find.widgetWithText(FilledButton, 'Supprimer'));
+      await tester.pumpAndSettle();
+
+      expect(find.text('Squat'), findsNothing);
+      expect(exerciseRepository.saved.single.metadata.deletedAt, isNotNull);
+    },
+  );
 }
 
 ExerciseUseCases _exerciseUseCases(_FakeExerciseRepository repository) {
   return ExerciseUseCases(
     repository: repository,
+    programRepository: _FakeProgramRepository(),
     clock: _FakeClock(DateTime.utc(2026, 7, 17, 12)),
     ids: _FakeIds(),
     originDeviceId: 'device-1',
   );
 }
 
+final class _FakeProgramRepository implements ProgramRepository {
+  @override
+  Future<Program?> findById(String id) async => null;
+
+  @override
+  Future<List<Program>> listActive() async => const [];
+
+  @override
+  Future<void> replaceExercises(Program program, DateTime deletedAt) async {}
+
+  @override
+  Future<void> save(Program program) async {}
+
+  @override
+  Future<void> saveExercise(ProgramExercise exercise) async {}
+}
+
 MediaUseCases _mediaUseCases(
   _FakeExerciseRepository exerciseRepository, {
   _FakeMediaStorage? storage,
@@ -174,16 +244,15 @@ MediaUseCases _mediaUseCases(
 }
 
 final class _FakeMediaPicker implements MediaSourcePicker {
-  const _FakeMediaPicker({this.imagePath, this.videoPath});
+  const _FakeMediaPicker({this.imagePath});
 
   final String? imagePath;
-  final String? videoPath;
 
   @override
   Future<String?> pickPath(MediaKind kind) async {
     return switch (kind) {
       MediaKind.image => imagePath,
-      MediaKind.video => videoPath,
+      MediaKind.video => null,
     };
   }
 }
@@ -235,7 +304,11 @@ final class _FakeExerciseRepository implements ExerciseRepository {
   Future<bool> isReferencedByProgram(String id) async => referenced;
 
   @override
-  Future<List<Exercise>> listActive() async => exercises;
+  Future<List<Exercise>> listActive() async {
+    return exercises
+        .where((exercise) => exercise.metadata.deletedAt == null)
+        .toList();
+  }
 
   @override
   Future<void> save(Exercise exercise) async {