refactor: modular local provider interfaces

Move package-type-specific local repo logic into provider packages via
optional interfaces, eliminating switch statements from handlers.

- provider.LocalUploader: ValidateUpload + UploadResponse
- provider.LocalIndexer: ServeLocalIndex + GenerateLocalIndex
- provider.FileStore: interface for querying local files (implemented
  by database.DB)

PyPI and Terraform providers now implement both interfaces. The local
handler and v1 proxy use type assertions to dispatch — adding a new
local repo type only requires implementing the interfaces in its
provider package, no handler changes needed.

local.go: 468 → 163 lines (removed all PyPI/Terraform specifics)
proxy.go: 211 → 136 lines (removed switch + helper methods)
engine.go: removed LocalIndexGenerator, uses provider.LocalIndexer

internal/server/server.go

@@ -63,7 +63,7 @@ func New(cfg *config.Config) (*Server, error) {
 
 	engine := proxy.NewEngine(db, redis, s3)
 	localHandler := v2.NewLocalHandler(db, s3)
-	virtEngine := virtual.NewEngine(db, engine, localHandler)
+	virtEngine := virtual.NewEngine(db, engine)
 	collector := gc.New(db, s3, 1*time.Hour)
 
 	s := &Server{