Fix Project and Tag model fields in PyPI proxy

Use correct model fields:
- Project: is_public, is_system, created_by (not visibility)
- Tag: add required created_by field

backend/app/pypi_proxy.py

@@ -488,7 +488,9 @@ async def pypi_download_file(
             system_project = Project(
                 name="_pypi",
                 description="System project for cached PyPI packages",
-                visibility="private",
+                is_public=False,
+                is_system=True,
+                created_by="pypi-proxy",
             )
             db.add(system_project)
             db.flush()
@@ -519,6 +521,7 @@ async def pypi_download_file(
                 package_id=package.id,
                 name=filename,
                 artifact_id=sha256,
+                created_by="pypi-proxy",
             )
             db.add(tag)