docs(api): add KOReader sync protocol documentation

- sync_progress.md - POST /api/sync/koreader/progress
- get_metadata.md - GET /api/sync/koreader/metadata/:uuid
- get_library.md - GET /api/sync/koreader/library
- sync_bookmarks.md - POST /api/sync/koreader/bookmarks

KOReader device sync endpoints with device authentication
for progress, metadata, library, and bookmarks

docs/developer/api/conflicts/resolve_conflict.md

@@ -0,0 +1,50 @@
+# Resolve Conflict
+
+Resolve a specific conflict by choosing which version to keep.
+
+**Endpoint**: `POST /api/conflicts/:id/resolve`
+**Auth**: Required
+**Content-Type**: `application/json`
+
+## Path Parameters
+
+| Parameter | Type | Required | Description |
+|-----------|------|-----------|-------------|
+| id | string (UUID) | Yes | Conflict UUID |
+
+## Request Body
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| resolution | string | Yes | Resolution strategy: "device", "server", or "highest_progress" |
+
+### Example Request
+
+```json
+{
+  "resolution": "device"
+}
+```
+
+## Response (200 OK)
+
+```json
+{
+  "message": "Conflict resolved successfully",
+  "conflict_id": "uuid",
+  "resolution": "device"
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 400 | Invalid resolution strategy |
+| 401 | Invalid or expired token |
+| 404 | Conflict not found |
+| 400 | Conflict already resolved |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->