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/koreader/push_stats.md

@@ -31,4 +31,6 @@ stored watermark becomes a reading_history session.
 }
 ```
 
-UNVERIFIED: the request body also carries the koreader book reference fields (md5, title, etc.) used to resolve the book; field set not copied here.
+The request body also embeds the koreader book reference used to resolve
+the book — fields (all optional, resolved via the same priority chain as
+progress sync): `uuid`, `sha256`, `title`, `authors` (string array).