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/database/local_files.go

@@ -8,6 +8,8 @@ import (
 
 	"github.com/jackc/pgx/v5"
 	"github.com/jackc/pgx/v5/pgconn"
+
+	"git.unkin.net/unkin/artifactapi/internal/provider"
 )
 
 type LocalFile struct {
@@ -122,6 +124,22 @@ func (db *DB) ListLocalFilePackages(ctx context.Context, repoName string) ([]str
 	return packages, rows.Err()
 }
 
+func (db *DB) ListFilesByPrefix(ctx context.Context, repoName, prefix string) ([]provider.FileEntry, error) {
+	files, err := db.ListLocalFilesByPrefix(ctx, repoName, prefix)
+	if err != nil {
+		return nil, err
+	}
+	result := make([]provider.FileEntry, len(files))
+	for i, f := range files {
+		result[i] = provider.FileEntry{FilePath: f.FilePath, ContentHash: f.ContentHash}
+	}
+	return result, nil
+}
+
+func (db *DB) ListPackages(ctx context.Context, repoName string) ([]string, error) {
+	return db.ListLocalFilePackages(ctx, repoName)
+}
+
 func (db *DB) DeleteLocalFile(ctx context.Context, repoName, filePath string) error {
 	_, err := db.Pool.Exec(ctx, `DELETE FROM local_files WHERE repo_name = $1 AND file_path = $2`, repoName, filePath)
 	return err