docs(stats): Claude-side pinning — all 4 stats endpoints + UNVERIFIED sweep to zero

Stats docs written handler/service-first (summary/this-week/book-stats
new; library-usage frontmatter'd — its body was already accurate).
Registry: 4 entries on services.UserSummaryStats/WeekSummaryStats/
LibraryUsageStats/BookStats. Family-lens note: library-usage 403s an
explicit library_id outside the visible set (431fec0 rule).

UNVERIFIED sweep (7 -> 0), each resolved against the handler:
- device-usage takes NO date params; popular-books takes only ?limit
  (the 'standard analytics date-range params' claim was wrong)
- link_book media_item_id: no validate tag but service-always-written
- push_stats book-ref field set documented (uuid/sha256/title/authors)
- filter_count: confirmed ParseMediaSearchParams machinery + 400 on bad
  filter + 403 visibility
- facets values element shape pinned: {value, display?, count}
- get_config: dynamic key set IS the contract (reworded, no marker)

make test 9/9.

docs/developer/api/media-items/get_facets.md

@@ -28,7 +28,10 @@ Two modes, both returning small JSON objects:
 }
 ```
 
-UNVERIFIED: exact element shape of `values` (service-returned rows).
+`values` elements are `{value, display?, count}` rows (`FacetValue`):
+`value` is the raw value that rides the filter param, `display` is the
+prettified form when it differs (language codes → names; absent means the
+client shows `value`), `count` is the number of matching items.
 
 ## Query parameters