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>

test/application/use_cases_test.dart

@@ -177,6 +177,113 @@ void main() {
     );
   });
 
+  test('Exercise use case rejects more than eight steps', () async {
+    final useCase = _exerciseUseCase(_FakeExerciseRepository());
+
+    await expectLater(
+      useCase.create(
+        name: 'Complexe',
+        hasTimeMeasure: false,
+        hasRepsMeasure: true,
+        hasScoreMeasure: false,
+        defaultTargetReps: 1,
+        steps: [
+          for (var index = 0; index < 9; index++)
+            _step(id: 'step-$index', position: index),
+        ],
+      ),
+      throwsA(isA<DomainException>()),
+    );
+  });
+
+  test('Exercise use case rejects non-contiguous step positions', () async {
+    final useCase = _exerciseUseCase(_FakeExerciseRepository());
+
+    await expectLater(
+      useCase.create(
+        name: 'Complexe',
+        hasTimeMeasure: false,
+        hasRepsMeasure: true,
+        hasScoreMeasure: false,
+        defaultTargetReps: 1,
+        steps: [
+          _step(id: 'step-1', position: 1),
+          _step(id: 'step-0', position: 0),
+        ],
+      ),
+      throwsA(isA<DomainException>()),
+    );
+  });
+
+  test('Exercise step validation rejects invalid basic fields', () {
+    expect(() => _step(name: ''), throwsA(isA<DomainException>()));
+    expect(() => _step(defaultTargetValue: 0), throwsA(isA<DomainException>()));
+  });
+
+  test('Exercise step validation rejects disabled score values', () {
+    expect(
+      () => _step(hasScore: false, scoreLabel: 'Charge'),
+      throwsA(isA<DomainException>()),
+    );
+    expect(
+      () => _step(hasScore: false, defaultTargetScore: 0),
+      throwsA(isA<DomainException>()),
+    );
+  });
+
+  test(
+    'Exercise step manual score validation accepts zero and rejects negative',
+    () {
+      final step = _step(
+        hasScore: true,
+        scoreLabel: 'Charge',
+        scoreUnit: 'kg',
+        defaultTargetScore: 0,
+      );
+
+      expect(step.defaultTargetScore, 0);
+      expect(
+        () => _step(
+          hasScore: true,
+          scoreLabel: 'Charge',
+          scoreUnit: 'kg',
+          defaultTargetScore: -1,
+        ),
+        throwsA(isA<DomainException>()),
+      );
+      expect(
+        () => _step(hasScore: true, scoreLabel: 'Charge'),
+        throwsA(isA<DomainException>()),
+      );
+    },
+  );
+
+  test('Exercise step stopwatch score validation rejects manual fields', () {
+    final step = _step(
+      hasScore: true,
+      scoreInputMode: ScoreInputMode.stopwatch,
+      defaultTargetScoreTimeMs: 12000,
+    );
+
+    expect(step.defaultTargetScoreTimeMs, 12000);
+    expect(
+      () => _step(
+        hasScore: true,
+        scoreInputMode: ScoreInputMode.stopwatch,
+        defaultTargetScoreTimeMs: 0,
+      ),
+      throwsA(isA<DomainException>()),
+    );
+    expect(
+      () => _step(
+        hasScore: true,
+        scoreInputMode: ScoreInputMode.stopwatch,
+        scoreLabel: 'Charge',
+      ),
+      throwsA(isA<DomainException>()),
+    );
+  });
+
   test('Exercise image gallery rejects a sixth image', () async {
     final repository = _FakeExerciseRepository()
       ..exercise = Exercise(
@@ -959,6 +1066,34 @@ void main() {
   });
 }
 
+ExerciseStep _step({
+  String id = 'step-1',
+  int position = 0,
+  String name = 'Phase',
+  ExerciseStepType type = ExerciseStepType.time,
+  int defaultTargetValue = 30,
+  bool hasScore = false,
+  ScoreInputMode scoreInputMode = ScoreInputMode.manual,
+  String? scoreLabel,
+  String? scoreUnit,
+  double? defaultTargetScore,
+  int? defaultTargetScoreTimeMs,
+}) {
+  return ExerciseStep(
+    id: id,
+    position: position,
+    name: name,
+    type: type,
+    defaultTargetValue: defaultTargetValue,
+    hasScore: hasScore,
+    scoreInputMode: scoreInputMode,
+    scoreLabel: scoreLabel,
+    scoreUnit: scoreUnit,
+    defaultTargetScore: defaultTargetScore,
+    defaultTargetScoreTimeMs: defaultTargetScoreTimeMs,
+  );
+}
+
 EntityMetadata _metadata(String id) {
   return EntityMetadata(
     id: id,