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/system/get_config.md

@@ -21,7 +21,7 @@ Returns all system-wide configuration key/value pairs (backed by the
 }
 ```
 
-UNVERIFIED: exact config key set is arbitrary (admin-defined); example shows common keys only.
+The key set is admin-defined and intentionally open — the endpoint emits\nwhatever config keys exist in `system_settings`. Treat the shape as a\ndynamic string-keyed map; do not code against a fixed key list.
 
 ## Errors