feat: virtual PyPI repos can merge local + remote members

The virtual engine now detects local members (repo_type=local) and
generates their package index in-memory instead of trying to fetch
from a non-existent upstream.

- MemberIndex gains RepoType field so mergers use correct URL prefix
  (/api/v1/local/ vs /api/v1/remote/)
- Virtual engine accepts a LocalIndexGenerator interface for producing
  local PyPI indexes
- LocalHandler implements GeneratePyPIPackageHTML for reuse by both
  the direct serving path and the virtual merger
- Includes local PyPI upload support (cherry-picked from benvin/local-pypi)

Tested e2e: local wheel upload + virtual merge + uv pip install from
both direct local and virtual URLs

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, "/"))
 			}