fix(arch-043,arch-052): deterministic system tenant lookup; async-safe file metadata

app/core/db/__init__.py

@@ -355,6 +355,23 @@ async def close_engine() -> None:
     _migration_session_factory = None
 
 
+async def get_system_tenant(db: AsyncSession):
+    """Return the well-known system tenant, or ``None`` if it does not exist.
+
+    Resolves by configured slug (``settings.system_tenant_slug``, default
+    ``"default"`` as created by ``scripts/seed_admin.py``) instead of an
+    arbitrary first row, so multi-tenant databases stay deterministic.
+    """
+    from sqlalchemy import select
+
+    from app.config import get_settings
+    from app.models.tenant import Tenant  # lazy: models import this module's Base
+
+    slug = get_settings().system_tenant_slug
+    result = await db.execute(select(Tenant).where(Tenant.slug == slug).limit(1))
+    return result.scalar_one_or_none()
+
+
 def reset_engine_for_testing(engine: AsyncEngine) -> async_sessionmaker[AsyncSession]:
     """Replace all global engines with a test engine. Returns a session factory.