Fix PyPI proxy: use correct storage method and make project public

- Use storage.get_stream(s3_key) instead of non-existent get_artifact_stream()
- Make _pypi project public (is_public=True) so cached packages are visible

backend/app/pypi_proxy.py

@@ -373,7 +373,7 @@ async def pypi_download_file(
 
             # Stream from S3
             try:
-                content_stream = storage.get_artifact_stream(artifact.id)
+                content_stream = storage.get_stream(artifact.s3_key)
 
                 return StreamingResponse(
                     content_stream,
@@ -488,7 +488,7 @@ async def pypi_download_file(
             system_project = Project(
                 name="_pypi",
                 description="System project for cached PyPI packages",
-                is_public=False,
+                is_public=True,
                 is_system=True,
                 created_by="pypi-proxy",
             )