refactor: modular local provider interfaces (#52)

## Summary
Move package-type-specific local repo logic out of centralized handlers into provider packages via optional Go interfaces.

**New interfaces in `provider` package:**
- \`LocalUploader\`: \`ValidateUpload(filePath) → (storagePath, contentType, error)\` + \`UploadResponse(...)\`
- \`LocalIndexer\`: \`ServeLocalIndex(w, r, files, repoName, path) → bool\` + \`GenerateLocalIndex(ctx, files, repoName, path) → ([]byte, error)\`
- \`FileStore\`: \`ListFilesByPrefix\` + \`ListPackages\` (implemented by database.DB)

**Providers implement these interfaces:**
- PyPI: upload validation (wheel/sdist naming), simple index serving + generation
- Terraform: upload validation (provider zip naming), mirror protocol serving

**Handlers simplified to generic dispatch:**
- \`local.go\`: type-asserts to \`LocalUploader\`, falls back to generic upload
- \`proxy.go\`: type-asserts to \`LocalIndexer\`, falls back to raw file serving
- \`engine.go\`: type-asserts to \`LocalIndexer\` for local virtual members

Adding a new local repo type (e.g. RPM) = implement the interfaces in its provider package. Zero handler changes.

## Test plan
- [x] Build + unit tests pass
- [x] E2E: PyPI local upload → simple index → uv pip install (smoke test after refactor)

Reviewed-on: #52
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

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