The sidebar's filter chips/bounds now ADAPT to the active filters: /facets takes the
current filters and computes each group's available values with all the OTHER active
filters applied but not its own (standard faceted search). So picking e.g. IMDb rating
>=8 trims the genre/content-rating chips + year bounds to what's still reachable, and
picking a genre narrows them further — while a multi-select group (genres) still shows
options to add. Extracted _meta_filter_conds (shared by browse/unified/facets), and the
frontend passes the live filters to plexFacets (query keyed on them, previous chips kept
during refetch). Search text is intentionally not factored into facets.
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