feat(exercice): galerie de 5 images par exercice (ticket #35)

Étend le modèle Drift (tables.dart, app_database.dart/.g.dart), les
entités/use cases/repositories et exercise_library_screen.dart pour
supporter jusqu'à 5 images par exercice au lieu d'une seule. flutter
analyze propre, 63/63 tests verts, build APK debug validé.

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

lib/application/use_cases.dart

@@ -24,6 +24,7 @@ final class ExerciseUseCases {
     required String name,
     String? description,
     String? imageMediaId,
+    List<String> imageMediaIds = const [],
     String? videoMediaId,
     required bool hasTimeMeasure,
     required bool hasRepsMeasure,
@@ -51,7 +52,7 @@ final class ExerciseUseCases {
       metadata: _newMetadata(ids, originDeviceId, now),
       name: name,
       description: description,
-      imageMediaId: imageMediaId,
+      imageMediaIds: _resolveExerciseImageIds(imageMediaId, imageMediaIds),
       videoMediaId: videoMediaId,
       hasTimeMeasure: hasTimeMeasure,
       hasRepsMeasure: hasRepsMeasure,
@@ -81,6 +82,7 @@ final class ExerciseUseCases {
     required String name,
     String? description,
     String? imageMediaId,
+    Object? imageMediaIds = _useCaseUnchanged,
     String? videoMediaId,
     required bool hasTimeMeasure,
     required bool hasRepsMeasure,
@@ -125,7 +127,9 @@ final class ExerciseUseCases {
       metadata: exercise.metadata.touch(clock.now()),
       name: name,
       description: description,
-      imageMediaId: imageMediaId,
+      imageMediaIds: imageMediaIds == _useCaseUnchanged
+          ? _resolveExerciseImageIds(imageMediaId, exercise.imageMediaIds)
+          : imageMediaIds as List<String>,
       videoMediaId: videoMediaId,
       hasTimeMeasure: hasTimeMeasure,
       hasRepsMeasure: hasRepsMeasure,
@@ -151,7 +155,62 @@ final class ExerciseUseCases {
       throw const DomainException('Exercise not found.');
     }
     final updated = exercise.copyWith(
-      imageMediaId: mediaAssetId,
+      imageMediaIds: _appendExerciseImage(exercise.imageMediaIds, mediaAssetId),
+      metadata: exercise.metadata.touch(clock.now()),
+    );
+    await repository.save(updated);
+    return updated;
+  }
+
+  Future<Exercise> addImage({
+    required String exerciseId,
+    required String mediaAssetId,
+  }) async {
+    final exercise = await repository.findById(exerciseId);
+    if (exercise == null) {
+      throw const DomainException('Exercise not found.');
+    }
+    final updated = exercise.copyWith(
+      imageMediaIds: _appendExerciseImage(exercise.imageMediaIds, mediaAssetId),
+      metadata: exercise.metadata.touch(clock.now()),
+    );
+    await repository.save(updated);
+    return updated;
+  }
+
+  Future<Exercise> removeImage({
+    required String exerciseId,
+    required String mediaAssetId,
+  }) async {
+    final exercise = await repository.findById(exerciseId);
+    if (exercise == null) {
+      throw const DomainException('Exercise not found.');
+    }
+    final updated = exercise.copyWith(
+      imageMediaIds: exercise.imageMediaIds
+          .where((imageId) => imageId != mediaAssetId)
+          .toList(),
+      metadata: exercise.metadata.touch(clock.now()),
+    );
+    await repository.save(updated);
+    return updated;
+  }
+
+  Future<Exercise> reorderImages({
+    required String exerciseId,
+    required List<String> orderedMediaIds,
+  }) async {
+    final exercise = await repository.findById(exerciseId);
+    if (exercise == null) {
+      throw const DomainException('Exercise not found.');
+    }
+    if (!_sameImageSet(exercise.imageMediaIds, orderedMediaIds)) {
+      throw const DomainException(
+        'Reordered exercise images must match existing images.',
+      );
+    }
+    final updated = exercise.copyWith(
+      imageMediaIds: orderedMediaIds,
       metadata: exercise.metadata.touch(clock.now()),
     );
     await repository.save(updated);
@@ -275,7 +334,10 @@ final class MediaUseCases {
     }
     final updated = switch (media.kind) {
       MediaKind.image => exercise.copyWith(
-        imageMediaId: media.metadata.id,
+        imageMediaIds: _appendExerciseImage(
+          exercise.imageMediaIds,
+          media.metadata.id,
+        ),
         metadata: exercise.metadata.touch(clock.now()),
       ),
       MediaKind.video => exercise.copyWith(
@@ -1479,6 +1541,33 @@ List<WorkoutTemplateProgram> _repositionWorkoutTemplatePrograms(
   ];
 }
 
+List<String> _resolveExerciseImageIds(
+  String? imageMediaId,
+  List<String> imageMediaIds,
+) {
+  if (imageMediaId != null) {
+    return [imageMediaId];
+  }
+  return imageMediaIds;
+}
+
+List<String> _appendExerciseImage(List<String> currentIds, String mediaId) {
+  if (currentIds.contains(mediaId)) {
+    return currentIds;
+  }
+  if (currentIds.length >= 5) {
+    throw const DomainException('An exercise cannot have more than 5 images.');
+  }
+  return [...currentIds, mediaId];
+}
+
+bool _sameImageSet(List<String> left, List<String> right) {
+  if (left.length != right.length) {
+    return false;
+  }
+  return left.toSet().containsAll(right) && right.toSet().containsAll(left);
+}
+
 void _validateExerciseDefaultTargets({
   required bool hasTimeMeasure,
   required bool hasRepsMeasure,