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/collections/bulk_assign.md

@@ -1,3 +1,11 @@
+---
+method: POST
+path: /api/collections/:id/books
+auth: jwt
+area: collections
+---
+<!-- source: internal/handlers/collections.go:506 CollectionHandler.AddBooks -->
+
 # Bulk Assign Books
 
 Add multiple books to a collection at once.
@@ -20,7 +28,7 @@ Add multiple books to a collection at once.
 
 ### Example Request
 
-```json
+```json example-request
 {
   "book_ids": [
     "660e8400-e29b-41d4-a716-446655440000",
@@ -40,6 +48,22 @@ Books added to collection successfully. No response body.
 - This is more efficient than adding books individually
 - Maximum 1000 books per request
 
+```json example-response
+{
+  "results": [
+    {
+      "book_id": "660e8400-e29b-41d4-a716-446655440001",
+      "status": "added"
+    }
+  ],
+  "total": 1
+}
+```
+
+Each result carries the operation outcome for one book (`added`/`error`
+variants come from the per-book loop; malformed ids report their own
+status instead of failing the batch).
+
 ## Error Responses
 
 | Code | Description                         |