fix(fc2a): three unit-test failures — async fixture decorator, name convention, lazy celery includes

test_health.py:
  @pytest.fixture on an `async def` function is rejected by pytest-asyncio
  1.x strict mode. Switched to @pytest_asyncio.fixture.

backend/app/models/import_settings.py:
  My constraint name was 'ck_import_settings_singleton' and Base.metadata's
  naming convention applies 'ck_<table>_<name>' on top, so the final ORM
  name was 'ck_import_settings_ck_import_settings_singleton' (double prefix).
  The migration creates the DB constraint as 'ck_import_settings_singleton'
  via raw alembic, so they didn't match. Fix: bare name 'singleton' in the
  model → convention produces 'ck_import_settings_singleton', matching the
  migration's literal name.

tests/test_tasks_register.py:
  Celery's include=[...] parameter on the constructor is lazy — task
  modules aren't imported until a worker boots. The test only imported
  the Celery instance, so the @celery.task decorators in scan.py /
  import_file.py / thumbnail.py never ran. Fix: explicit `import` of
  those modules for side-effect at the top of the test file.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/models/import_settings.py

@@ -12,7 +12,9 @@ from .base import Base
 
 class ImportSettings(Base):
     __tablename__ = "import_settings"
-    __table_args__ = (CheckConstraint("id = 1", name="ck_import_settings_singleton"),)
+    # Bare constraint name — Base.metadata's naming convention applies the
+    # ck_<table>_<name> prefix, producing the final ck_import_settings_singleton.
+    __table_args__ = (CheckConstraint("id = 1", name="singleton"),)
 
     id: Mapped[int] = mapped_column(Integer, primary_key=True)
     import_scan_path: Mapped[str] = mapped_column(Text, nullable=False, default="/import")