refactor: rename standardsite platform to other
standardsite was a misnomer - it's a lexicon (site.standard.*), not a platform.
Documents using this lexicon that don't belong to a known platform (leaflet,
pckt, offprint) are now categorized as "other".
- extractor.zig: rename Platform.standardsite to Platform.other
- schema.zig: migration to rename existing records, update detection queries
- stats.zig: update sample query
- site/index.html: add "other" to platform filter, update empty state
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