feat: virtual PyPI repos can merge local + remote members (#51)

## Summary
- Virtual engine detects local members and generates indexes in-memory
- MemberIndex.RepoType drives correct URL prefix in merged output
- PyPI merger rewrites links to /api/v1/local/ or /api/v1/remote/ appropriately
- Includes local PyPI support (cherry-picked from #50)

## Test plan
- [x] Upload wheel to local PyPI → install from direct local URL
- [x] Create virtual with local + remote → install from virtual URL
- [x] Both paths produce correct absolute download URLs

Reviewed-on: #51
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

internal/virtual/merger.go

@@ -8,6 +8,7 @@ import (
 
 type MemberIndex struct {
 	RemoteName string
+	RepoType   models.RepoType
 	Body       []byte
 }