docs: add book-matching, collections, and OPDS API endpoints

Phase 2 part 6: Split advanced features endpoints
- Book Matching: search_books, link_book (manual and auto-link)
- Collections: INDEX.md linking to COLLECTIONS_API.md
- OPDS: feeds, acquisition, publication (OPDS 1.2 protocol)
- Include format conversion details (EPUB to KEPUB)

docs/api/book-matching/search_books.md

@@ -0,0 +1,56 @@
+# Search Books for Matching
+
+Query books to find potential matches for linking.
+
+**Endpoint**: `POST /api/sync/books/query`
+**Auth**: Required
+**Content-Type**: `application/json`
+
+## Request Body
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| identifiers | array | No | List of identifiers (ISBN, UUID) |
+| sha256 | string | No | SHA256 hash of book file |
+| title | string | No | Book title |
+| author | string | No | Book author |
+| file_size | integer | No | File size in bytes |
+
+### Example Request
+
+```json
+{
+  "identifiers": ["isbn:978-0345391802", "uuid:abc-123"],
+  "sha256": "a1b2c3d4e5f6abc123...",
+  "title": "The Hobbit",
+  "author": "J.R.R. Tolkien",
+  "file_size": 2456789
+}
+```
+
+## Response (200 OK)
+
+```json
+{
+  "matches": [
+    {
+      "media_item_id": "uuid-123",
+      "bookhoard_uuid": "uuid-123",
+      "confidence": 1.0,
+      "match_method": "uuid_match"
+    }
+  ],
+  "action": "auto_link"
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 400 | Invalid query parameters |
+| 401 | Invalid or expired token |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->