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/merger.go

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