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/bulk_resolve_conflicts.md

@@ -0,0 +1,48 @@
+# Bulk Resolve Conflicts
+
+Resolve multiple conflicts at once using a specified strategy.
+
+**Endpoint**: `POST /api/conflicts/bulk-resolve`
+**Auth**: Required
+**Content-Type**: `application/json`
+
+## Request Body
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| conflict_ids | array of UUID | Yes | Array of conflict UUIDs to resolve |
+| resolution | string | Yes | Resolution strategy: "device", "server", or "highest_progress" |
+
+### Example Request
+
+```json
+{
+  "conflict_ids": [
+    "550e8400-e29b-41d4-a716-446655440000",
+    "660e8400-e29b-41d4-a716-446655440001"
+  ],
+  "resolution": "highest_progress"
+}
+```
+
+## Response (200 OK)
+
+```json
+{
+  "message": "Conflicts resolved successfully",
+  "resolved_count": 2,
+  "failed_count": 0
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 400 | Invalid request body |
+| 401 | Invalid or expired token |
+| 400 | Invalid resolution strategy |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->