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/provider/provider.go

@@ -24,6 +24,26 @@ type Provider interface {
 	AuthHeaders(ctx context.Context, remote models.Remote) (http.Header, error)
 }
 
+type FileEntry struct {
+	FilePath    string
+	ContentHash string
+}
+
+type FileStore interface {
+	ListFilesByPrefix(ctx context.Context, repoName, prefix string) ([]FileEntry, error)
+	ListPackages(ctx context.Context, repoName string) ([]string, error)
+}
+
+type LocalUploader interface {
+	ValidateUpload(filePath string) (storagePath, contentType string, err error)
+	UploadResponse(storagePath, contentHash string, sizeBytes int64) map[string]any
+}
+
+type LocalIndexer interface {
+	ServeLocalIndex(w http.ResponseWriter, r *http.Request, files FileStore, repoName, path string) bool
+	GenerateLocalIndex(ctx context.Context, files FileStore, repoName, path string) ([]byte, error)
+}
+
 type IndexMerger interface {
 	MergeIndexes(members []MemberIndex, proxyBaseURL string) ([]byte, error)
 }