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/infrastructure/local/tables.dart

@@ -55,10 +55,6 @@ class Exercises extends SyncableTable {
 
   TextColumn get name => text().withLength(min: 1)();
   TextColumn get description => text().nullable()();
-  @ReferenceName('exerciseImageReferences')
-  TextColumn get imageMediaId =>
-      text().nullable().references(MediaAssets, #id)();
-
   @ReferenceName('exerciseVideoReferences')
   TextColumn get videoMediaId =>
       text().nullable().references(MediaAssets, #id)();
@@ -92,6 +88,22 @@ class Exercises extends SyncableTable {
   ];
 }
 
+class ExerciseImages extends SyncableTable {
+  @override
+  String get tableName => 'exercise_images';
+
+  TextColumn get exerciseId => text().references(Exercises, #id)();
+  TextColumn get mediaAssetId => text().references(MediaAssets, #id)();
+  IntColumn get position => integer()();
+
+  @override
+  List<String> get customConstraints => [
+    'UNIQUE (exercise_id, position)',
+    'UNIQUE (exercise_id, media_asset_id)',
+    'CHECK (position >= 0 AND position < 5)',
+  ];
+}
+
 class Programs extends SyncableTable {
   @override
   String get tableName => 'programs';