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/stats/get_library_usage.md

@@ -1,3 +1,11 @@
+---
+method: GET
+path: /api/stats/library-usage
+auth: jwt
+area: stats
+---
+<!-- source: internal/handlers/stats.go:88 StatsHandler.LibraryUsage -> services/statistics_service.go:586 LibraryUsageStats -->
+
 # Get Library Usage Statistics
 
 The `/stats` page's **Library tab** payload: media_items **inventory**
@@ -29,7 +37,7 @@ Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...
 
 ## Response (200 OK)
 
-```json
+```json example-response
 {
   "library_id": "8c8d0cee-1cca-4def-9a37-edad3a4d438b",
   "items": 743,