feat: add durable sync state transitions

Add state-transition methods to ActivityRepository for advancing activity
stages (mark_downloaded, mark_converted, mark_imported, mark_duplicate,
mark_failed) with proper retention of last_completed_stage on failure.
Add list_pending_for_user to filter activities for processing. Implement
SyncRunRepository for creating and finalizing sync runs with counts and
summary errors.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/conftest.py

@@ -8,8 +8,8 @@ from sqlalchemy.orm import Session, sessionmaker
 from sqlalchemy.pool import StaticPool
 
 from app.config import Settings
-from app.db.models import Base
-from app.db.repositories import ActivityRepository, UserRepository
+from app.db.models import Activity, Base, HealthState
+from app.db.repositories import ActivityRepository, SyncRunRepository, UserRepository
 from app.main import create_app
 
 
@@ -39,6 +39,11 @@ def activity_repository(db_session: Session) -> ActivityRepository:
     return ActivityRepository(db_session)
 
 
+@pytest.fixture
+def sync_run_repository(db_session: Session) -> SyncRunRepository:
+    return SyncRunRepository(db_session)
+
+
 @pytest.fixture
 def client(tmp_path: Path) -> TestClient:
     settings = Settings(
@@ -54,3 +59,23 @@ def client(tmp_path: Path) -> TestClient:
         yield TestClient(app)
     finally:
         app.state.db_engine.dispose()
+
+
+@pytest.fixture
+def seeded_activity(db_session: Session, user_repository: UserRepository, activity_repository: ActivityRepository) -> Activity:
+    user = user_repository.create(
+        name="Test User",
+        enabled=True,
+        health_state=HealthState.HEALTHY,
+        mywhoosh_email_enc="test@example.com",
+        mywhoosh_password_enc="password",
+        garmin_email_enc="test@garmin.com",
+        garmin_password_enc="garmin_password",
+    )
+    activity, _ = activity_repository.get_or_create_discovered(
+        user_id=user.id,
+        mywhoosh_activity_id="mw-test-123",
+        activity_name="Test Activity",
+        activity_timestamp=None,
+    )
+    return activity