docs(book-matching,collections): Beam batch 10 — 15 endpoints

WO3 sync internals resolved: the book-matching docs ARE the sync surface
(books/query, link-book, bulk/auto-link, unlinked-books, suggestions,
file-alias CRUD) + collections rule/test/bulk-assign/shelf-mapping
surface.

Dedupe resolved (Claude call): search_books.md declared
GET /api/books/match but its BODY documents POST /api/sync/books/query —
both routes exist and are distinct (query-param device protocol vs JSON
body plugin protocol). Repointed search_books.md frontmatter to the POST
route; get_book_matches.md keeps the GET.

Claude registry: 15 entries (one pre-existing link-book entry deduped).
Drift fixed by gate: auto-link request {confidence_threshold, limit} not
{device_id, threshold}; bulk-link link objects {unlinked_book_id,
media_item_id, confidence_score} not {device_id, book_id, ...};
responses {results,total,successful,failed}/{auto_linked,results}; added
missing example-responses (bare delete, remove-rule is_smart-after
semantics, bulk-assign per-book results). make test 9/9.

docs/developer/api/book-matching/link_book.md

@@ -1,7 +1,45 @@
+---
+method: POST
+path: /api/devices/:deviceId/sync/link-book
+auth: jwt
+area: book-matching
+---
+<!-- source: internal/handlers/matching.go:57 MatchingHandler.LinkBook -->
+
 # Link Book
 
 Link a device book to a Bookhoard media item. Supports bulk linking.
 
+## Example request
+
+```json example-request
+{
+  "device_file": {
+    "file_path": "/books/hobbit.epub",
+    "sha256": "abc123...",
+    "title": "The Hobbit"
+  },
+  "media_item_id": "550e8400-e29b-41d4-a716-446655440000",
+  "confidence_score": 1.0
+}
+```
+
+## Example response
+
+```json example-response
+{
+  "status": "linked",
+  "device_file_alias": {
+    "id": "550e8400-e29b-41d4-a716-446655440000",
+    "media_item_id": "550e8400-e29b-41d4-a716-446655440000",
+    "device_id": "550e8400-e29b-41d4-a716-446655440000",
+    "file_path": "/books/hobbit.epub",
+    "file_sha256": "abc123...",
+    "confidence_score": 1.0
+  }
+}
+```
+
 **Endpoint**: `POST /api/sync/bulk-link-books` or `POST /api/sync/auto-link-books`
 **Auth**: Required
 **Content-Type**: `application/json`