Remove deprecated anonymous download creation endpoint

[oskarhurst]

The deprecated POST /api/deprecated/anonymous_downloads route allowed callers to create packaging jobs without authentication using a shared anonymous user ID. Remove that route so download creation uses the existing authenticated /api/downloads and /api/my_downloads endpoints.

Delete the now-unused AttachAnonymousUserInfo middleware and update the related identity comments. Existing public download status, file-serving, and packager-request routes retain their current behavior. Historical download records are not changed.

Validation

• API Docker image built successfully.
• Six HTTP checks against a temporary API container with AUTH_ENVIRONMENT=TEST passed:
  • Removed route: 401 without credentials (existing API auth fallback); 404 with a valid application key.
  • /api/downloads and /api/my_downloads: 401 without credentials.
  • Both supported routes: 400 for malformed JSON with a valid application key, confirming they still reach the creation handler.
• The temporary API used no live database; malformed request bodies prevented job creation. This verifies routing and authentication, not end-to-end packaging.
• No references to the removed route or middleware remain in the API, API specification, or regression collections; git diff --check passed.