docs(media-items,collections,koreader,admin): Beam batch 11 — final backfill

All remaining endpoint docs frontmatter'd: media-item bulk ops + audio
surface, collections book-listing/preview, koreader rating/review/stats
plugin protocol, hash-conflicts JSON list.

Scope call (Claude): resolve_hash_conflict.md DELETED — the resolve POST
renders an htmx fragment (renderResolved), a web-admin flow under rule 3;
only the GET list is a JSON API. update_media_review skipped by Beam
(correct): ratings/upsert_media_review.md already covers the PUT.

Registry: 16 entries. koreader rating/review/stats all bind
koreaderBookRef (query or body) — odd=half-star rating scale; audio track
endpoint serves raw bytes (ServeContent, Range) — response nil. Fixed
drift: deleted_annotations doc's secondary restore/purge fences
unlabeled (gate validates labeled blocks against the ONE frontmatter
route); chapters example labeled. UNVERIFIED: 1 (koreader book-ref
fields, push_stats). make test 9/9.

docs/developer/api/koreader/set_rating.md

@@ -0,0 +1,29 @@
+---
+method: POST
+path: /api/sync/koreader/rating
+auth: device-token
+area: koreader
+---
+<!-- source: internal/handlers/koreader.go:1740 KOReaderHandler.SetKOReaderRating -->
+
+# Set KOReader Rating
+
+Sets (or clears, when rating is 0) the rating for a book.
+
+## Example request
+
+```json example-request
+{
+  "rating": 8
+}
+```
+
+## Example response
+
+```json example-response
+{
+  "rating": 8
+}
+```
+
+A rating of 0 clears the stored rating and returns `{"rating": null}`.