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

backend/app/main.py

@@ -11,6 +11,7 @@ from slowapi.errors import RateLimitExceeded
 from .config import get_settings
 from .database import init_db, SessionLocal
 from .routes import router
+from .pypi_proxy import router as pypi_router
 from .seed import seed_database
 from .auth import create_default_admin
 from .rate_limit import limiter
@@ -65,6 +66,7 @@ app.add_exception_handler(RateLimitExceeded, _rate_limit_exceeded_handler)
 
 # Include API routes
 app.include_router(router)
+app.include_router(pypi_router)
 
 # Serve static files (React build) if the directory exists
 static_dir = os.path.join(os.path.dirname(__file__), "..", "..", "frontend", "dist")