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/pypi_merger.go

@@ -36,8 +36,13 @@ func (m *PyPIMerger) MergeIndexes(members []MemberIndex, proxyBaseURL string) ([
 			}
 
 			if proxyBaseURL != "" && href != "" {
-				href = fmt.Sprintf("%s/api/v1/remote/%s/%s",
+				routePrefix := "remote"
+				if member.RepoType == "local" {
+					routePrefix = "local"
+				}
+				href = fmt.Sprintf("%s/api/v1/%s/%s/%s",
 					strings.TrimRight(proxyBaseURL, "/"),
+					routePrefix,
 					member.RemoteName,
 					strings.TrimLeft(href, "/"))
 			}