Separate legacy filesystem and database code paths: Blob DELETE

Summary

Refactor the blob DELETE handler in registry/handlers/blob.go to separate the legacy filesystem code path from the database code path, as part of the effort to improve maintainability described in &20922.

Context

Route: DELETE /v2/<name>/blobs/<digest>

Current file: registry/handlers/blob.go

Handler method: DeleteBlob

Current State

The handler currently uses conditional branching to switch between database and filesystem metadata backends. This creates cognitive overhead and intermingles actively developed database code with legacy filesystem code.

Goal

Separate the two code paths following the pattern established in !2766 (closed):

  1. Extract legacy filesystem logic into a dedicated method in a legacy_*.go file
  2. Main handler method contains only database logic
  3. Backend selection occurs at the dispatcher level

Acceptance Criteria

  • Legacy filesystem logic isolated in legacy_blob.go
  • Main handler contains only database logic
  • Dispatcher selects appropriate handler based on backend configuration
  • API conformance tests pass for both database and legacy configurations

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