- Implement PEP 503 Simple API endpoints for pip compatibility:
- GET /pypi/simple/ - package index
- GET /pypi/simple/{package}/ - version list with rewritten links
- GET /pypi/simple/{package}/{filename} - download with auto-caching
- Improve upstream sources table UI:
- Center text under column headers
- Remove separate Source column, show ENV badge inline with name
- Make Test/Edit buttons more prominent with secondary button style
