docs(api): document the KOReader resolve endpoint

Add koreader/resolve_book.md for GET /api/sync/koreader/resolve, list
the endpoint in the API reference, and describe the resolve-then-pull-
then-push linking flow in the KOReader protocol page — including why a
device pushing to bootstrap its identity creates progress conflicts for
books already mid-read from other sources.

docs/developer/api/sync/koreader-protocol.md

@@ -94,6 +94,41 @@ hash differs from the primary format's hash.
 }
 ```
 
+## Book Resolution (UUID lookup)
+
+**Endpoint**: `GET /api/sync/koreader/resolve?sha256={hash}`
+**Auth**: Device token required
+
+Read-only lookup mapping a file SHA-256 to the book's UUID (format-aware,
+same `BookResolver` path as the progress push). Devices call this on the
+first open of a newly downloaded book to learn the UUID **before** their
+first pull. Full details: [Resolve Book](../koreader/resolve_book.md).
+
+This matters for conflict avoidance: a device that pushes to bootstrap its
+identity transmits its current (first-page) position, which the server
+treats as a real progress update — overwriting/conflicting with genuine
+mid-read progress from other sources. Resolve, then pull, then push.
+
+### Example Request
+
+```http
+GET /api/sync/koreader/resolve?sha256=e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855
+Authorization: Bearer device-token
+```
+
+### Response (200 OK)
+
+```json
+{
+  "book_uuid": "book-uuid",
+  "sha256": "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",
+  "title": "Book Title",
+  "author": "Author Name"
+}
+```
+
+404 when no book in the library matches the hash.
+
 ## KOReader Metadata Fetch
 
 **Endpoint**: `GET /api/sync/koreader/metadata/{book_uuid}`