feat: add path field for proper document URL building
site.standard.document records have a `path` field (e.g., "/001") that
should be used with the publication's base URL to build the full document
URL. Previously was incorrectly using rkey which caused 404s.
- extract path field from records in extractor.zig
- store path in documents table via schema migration
- return path in search results
- frontend uses basePath + path when path is available
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>