Add a facets endpoint that returns per-tag channel counts for the current filter
context (scope, channel, date, content type, search, watch state, and the other
category's tags). Each category is counted with its own selections ignored —
standard drill-down faceting — by a new exclude_tag_category param threaded into
_filtered_query, so selecting one topic doesn't zero out the other topics. Count
is distinct channels with a matching video, keeping the channel-count chip
semantics. Reuses the feed's filter query so both stay in lockstep.
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