Add transparent PyPI proxy and improve upstream sources UI

- 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

CHANGELOG.md

@@ -6,6 +6,19 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+### Added
+- Added transparent PyPI proxy implementing PEP 503 Simple API
+  - `GET /pypi/simple/` - package index (proxied from upstream)
+  - `GET /pypi/simple/{package}/` - version list with rewritten download links
+  - `GET /pypi/simple/{package}/{filename}` - download with automatic caching
+  - Allows `pip install --index-url https://orchard.../pypi/simple/ <package>`
+  - Artifacts cached on first access through configured upstream sources
+
+### Changed
+- Upstream sources table text is now centered under column headers
+- ENV badge now appears inline with source name instead of separate column
+- Test and Edit buttons now have more prominent button styling
+
 ### Fixed
 - Fixed purge_seed_data crash when deleting access permissions - was comparing UUID to VARCHAR column (#107)