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/koreader/resolve_book.md

@@ -0,0 +1,50 @@
+# Resolve Book
+
+Map a book's file SHA-256 to its Bookhoard UUID without touching progress
+state. Used by devices to link a freshly downloaded book before their first
+pull, so the device's first-page position is never pushed (which would
+conflict with server-side progress for books already mid-read).
+
+**Endpoint**: `GET /api/sync/koreader/resolve`
+**Auth**: Required (Device authentication)
+
+## Query Parameters
+
+| Parameter | Type   | Required | Description                          |
+| --------- | ------ | -------- | ------------------------------------ |
+| sha256    | string | Yes      | File content hash (64 hex characters) |
+
+Resolution is format-aware: the hash is checked against both
+`media_items.file_sha256` and `media_item_formats.file_sha256`, so a
+converted file (KEPUB/PDF) matches its media item too.
+
+## Device Authentication
+
+This endpoint requires device authentication (not user JWT). Devices
+authenticate using their device credentials.
+
+### Example Request
+
+```http
+GET /api/sync/koreader/resolve?sha256=e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855
+Authorization: Bearer {device_token}
+```
+
+## Response (200 OK)
+
+```json
+{
+  "book_uuid": "550e8400-e29b-41d4-a716-446655440000",
+  "sha256": "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",
+  "title": "Book Title",
+  "author": "Author Name"
+}
+```
+
+## Error Responses
+
+| Code | Description                                  |
+| ---- | -------------------------------------------- |
+| 400  | Missing or malformed `sha256` parameter      |
+| 401  | Device authentication failed                 |
+| 404  | No book in the library matches the given hash |