feat(exercice): modèle domain + Drift pour exercices à étapes (ticket #56)

Étend le modèle Drift (tables.dart, app_database.dart/.g.dart,
migration schemaVersion 7→8) et la couche application (entités, use
cases, repositories) pour supporter des exercices composés de
plusieurs étapes. flutter pub get OK, build_runner OK, flutter
analyze propre (mêmes 8 infos préexistantes), 87/87 tests verts,
build APK debug validé. Premier ticket du chantier "Exercice à
plusieurs étapes" (#54).

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

lib/infrastructure/local/app_database.dart

@@ -14,6 +14,7 @@ part 'app_database.g.dart';
     ChangeLogEntries,
     Exercises,
     ExerciseImages,
+    ExerciseSteps,
     MediaAssets,
     ProgramExercises,
     Programs,
@@ -37,7 +38,7 @@ final class AppDatabase extends _$AppDatabase {
   }
 
   @override
-  int get schemaVersion => 7;
+  int get schemaVersion => 8;
 
   @override
   MigrationStrategy get migration {
@@ -73,6 +74,9 @@ final class AppDatabase extends _$AppDatabase {
         if (from < 7) {
           await _migrateToSchema7();
         }
+        if (from < 8) {
+          await _migrateToSchema8(migrator);
+        }
         await _createIndexes();
       },
       beforeOpen: (details) async {
@@ -109,6 +113,10 @@ final class AppDatabase extends _$AppDatabase {
       'CREATE INDEX IF NOT EXISTS idx_exercise_images_exercise_id '
       'ON exercise_images (exercise_id)',
     );
+    await customStatement(
+      'CREATE INDEX IF NOT EXISTS idx_exercise_steps_exercise_id_position '
+      'ON exercise_steps (exercise_id, position)',
+    );
     await customStatement(
       'CREATE INDEX IF NOT EXISTS idx_workout_template_programs_template_id '
       'ON workout_template_programs (workout_template_id)',
@@ -162,6 +170,7 @@ const _syncableTableNames = [
   'active_workout_sessions',
   'exercises',
   'exercise_images',
+  'exercise_steps',
   'media_assets',
   'program_exercises',
   'programs',
@@ -270,4 +279,12 @@ extension on AppDatabase {
       'REFERENCES media_assets(id)',
     );
   }
+
+  Future<void> _migrateToSchema8(Migrator migrator) async {
+    await migrator.createTable(exerciseSteps);
+    await customStatement(
+      'ALTER TABLE program_exercises ADD COLUMN '
+      'exercise_steps_snapshot_json TEXT',
+    );
+  }
 }