Extend the composable gallery filter with platform / untagged / no_artist /
date_from / date_to, AND-composed with the existing tag/artist/media/sort
params and threaded through scroll, timeline, and jump_cursor.
Add GalleryService.facets() + GET /api/gallery/facets returning live counts
scoped to the current filter with per-group minus-self semantics: platform
counts (COUNT(DISTINCT image) incl. a null unsourced bucket), curation-flag
counts (untagged / no_artist), and effective_date min/max bounds. The
UNSOURCED_PLATFORM sentinel makes filesystem-imported content reachable via
the platform facet.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
