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/devices/link_book.md

@@ -48,7 +48,7 @@ confidence score. Used by the book-matching flow.
 | device_file.file_path | string | no | path on the device |
 | device_file.sha256 | string | no | file content hash |
 | device_file.title | string | no | device-side title |
-| media_item_id | string (uuid) | yes | `services.LinkBookRequest` declares it as uuid; the gate only checks the Bind struct — UNVERIFIED: confirm required tag on LinkBookRequest fields |
+| media_item_id | string (uuid) | yes | no validate tag, but the service always writes it (upsert keyed on media item) — omitting it orphans the alias |
 | confidence_score | number | no | match confidence (0-1) |
 
 ## Errors