feat(media): gestion des médias locaux

Ajoute le stockage local des médias (infrastructure/local/local_media_storage.dart)
et les use cases/ports associés. flutter analyze propre, 7/7 tests
unitaires verts, build APK debug validé.

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

lib/application/use_cases.dart

@@ -46,6 +46,38 @@ final class ExerciseUseCases {
 
   Future<void> save(Exercise exercise) => repository.save(exercise);
 
+  Future<Exercise> attachImage({
+    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(
+      imageMediaId: mediaAssetId,
+      metadata: exercise.metadata.touch(clock.now()),
+    );
+    await repository.save(updated);
+    return updated;
+  }
+
+  Future<Exercise> attachVideo({
+    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(
+      videoMediaId: mediaAssetId,
+      metadata: exercise.metadata.touch(clock.now()),
+    );
+    await repository.save(updated);
+    return updated;
+  }
+
   Future<Exercise> archive(String id) async {
     final exercise = await repository.findById(id);
     if (exercise == null) {
@@ -57,6 +89,91 @@ final class ExerciseUseCases {
   }
 }
 
+final class MediaUseCases {
+  const MediaUseCases({
+    required this.mediaRepository,
+    required this.exerciseRepository,
+    required this.storage,
+    required this.clock,
+    required this.ids,
+    required this.originDeviceId,
+  });
+
+  final MediaAssetRepository mediaRepository;
+  final ExerciseRepository exerciseRepository;
+  final LocalMediaStorage storage;
+  final Clock clock;
+  final IdGenerator ids;
+  final String originDeviceId;
+
+  Future<MediaAsset> importMedia({
+    required String sourcePath,
+    required MediaKind kind,
+  }) async {
+    final now = clock.now();
+    final id = ids.newId();
+    final stored = await storage.importFile(
+      sourcePath: sourcePath,
+      kind: kind,
+      stableFileName: id,
+    );
+    final asset = MediaAsset(
+      metadata: EntityMetadata(
+        id: id,
+        createdAt: now,
+        updatedAt: now,
+        originDeviceId: originDeviceId,
+      ),
+      kind: kind,
+      localUri: stored.localUri,
+      mimeType: stored.mimeType,
+      sizeBytes: stored.sizeBytes,
+      width: stored.width,
+      height: stored.height,
+      durationMs: stored.durationMs,
+    );
+    await mediaRepository.save(asset);
+    return asset;
+  }
+
+  Future<Exercise> attachToExercise({
+    required String exerciseId,
+    required String mediaAssetId,
+  }) async {
+    final exercise = await exerciseRepository.findById(exerciseId);
+    if (exercise == null) {
+      throw const DomainException('Exercise not found.');
+    }
+    final media = await mediaRepository.findById(mediaAssetId);
+    if (media == null) {
+      throw const DomainException('Media asset not found.');
+    }
+    final updated = switch (media.kind) {
+      MediaKind.image => exercise.copyWith(
+        imageMediaId: media.metadata.id,
+        metadata: exercise.metadata.touch(clock.now()),
+      ),
+      MediaKind.video => exercise.copyWith(
+        videoMediaId: media.metadata.id,
+        metadata: exercise.metadata.touch(clock.now()),
+      ),
+    };
+    await exerciseRepository.save(updated);
+    return updated;
+  }
+
+  Future<int> deleteOrphanedManagedFiles() async {
+    final activeAssets = await mediaRepository.listActive();
+    final referencedUris = activeAssets.map((asset) => asset.localUri).toSet();
+    final managedUris = await storage.listManagedLocalUris();
+    final orphanedUris = managedUris.difference(referencedUris);
+    for (final uri in orphanedUris) {
+      await storage.deleteByLocalUri(uri);
+    }
+    return orphanedUris.length;
+  }
+}
+
 final class ProgramUseCases {
   const ProgramUseCases({
     required this.programRepository,