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/admin/hash_conflicts_api.md

@@ -0,0 +1,52 @@
+---
+method: GET
+path: /api/admin/hash-conflicts
+auth: jwt+admin
+area: admin
+---
+<!-- source: internal/handlers/hash_conflicts.go:69 HashConflictsHandler.ListHashConflicts -->
+
+# Hash Conflicts (API)
+
+Lists pending content-duplicate groups (same library + SHA-256 at different
+paths) with per-item usage counts.
+
+## Example response
+
+```json example-response
+{
+  "conflicts": [
+    {
+      "id": "uuid",
+      "library_id": "uuid",
+      "library_name": "Manga",
+      "sha256": "abc123...",
+      "created_at": "2026-10-24T13:57:01Z",
+      "items": [
+        {
+          "id": "uuid",
+          "title": "Book Title",
+          "author": "Author",
+          "file_path": "/path/to/book.epub",
+          "file_size": 12345,
+          "created_at": "2026-10-24T13:57:01Z",
+          "progress_count": 0,
+          "highlight_count": 0,
+          "bookmark_count": 0,
+          "note_count": 0,
+          "collection_count": 0
+        }
+      ]
+    }
+  ],
+  "total": 1
+}
+```
+
+## Errors
+
+| Status | Body | When |
+| ------ | ---- | ---- |
+| 401 | `{"error": "unauthorized"}` | no/invalid JWT |
+| 403 | `{"error": "admin access required"}` | non-admin |
+| 500 | `{"error": "failed to list hash conflicts"}` | unexpected server error |