fix: use HTTP site URL as base_path for standard.site documents
standard.site documents store the origin URL (e.g., "https://attoshi.com")
in publication_uri, but the indexer only resolves base_path from AT-URIs
via the publications table. When publication_uri is an HTTP URL, the lookup
fails silently and base_path stays empty, breaking frontend links.
Add a fallback: if base_path is still empty and publication_uri starts with
http(s)://, strip the scheme and use the remainder as base_path.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>