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/ports.dart

@@ -21,6 +21,41 @@ abstract interface class ExerciseRepository {
   Future<void> save(Exercise exercise);
 }
 
+final class StoredMediaFile {
+  const StoredMediaFile({
+    required this.localUri,
+    required this.mimeType,
+    required this.sizeBytes,
+    this.width,
+    this.height,
+    this.durationMs,
+  });
+
+  final String localUri;
+  final String? mimeType;
+  final int sizeBytes;
+  final int? width;
+  final int? height;
+  final int? durationMs;
+}
+
+abstract interface class LocalMediaStorage {
+  Future<StoredMediaFile> importFile({
+    required String sourcePath,
+    required MediaKind kind,
+    required String stableFileName,
+  });
+
+  Future<Set<String>> listManagedLocalUris();
+  Future<void> deleteByLocalUri(String localUri);
+}
+
+abstract interface class MediaAssetRepository {
+  Future<MediaAsset?> findById(String id);
+  Future<List<MediaAsset>> listActive();
+  Future<void> save(MediaAsset mediaAsset);
+}
+
 abstract interface class ProgramRepository {
   Future<Program?> findById(String id);
   Future<void> save(Program program);