docs(api): add sync queue management documentation

- get_device_queue_stats.md - Get queue statistics for device
- list_device_queue_items.md - List queue items with filtering
- retry_queue_item.md - Retry failed queue items
- delete_queue_item.md - Delete queue items
- clear_device_queue.md - Clear entire device queue
- list_all_queue_items.md - List all queue items (admin)

Documents sync queue management for handling failed/queued operations
between devices and server

docs/developer/api/devices/reject_device_registration.md

@@ -0,0 +1,46 @@
+# Reject Device Registration
+
+Reject a pending device registration request.
+
+**Endpoint**: `POST /api/devices/reject/:registration_id`
+**Auth**: Required (Admin only)
+
+## Path Parameters
+
+| Parameter | Type | Required | Description |
+|-----------|------|-----------|-------------|
+| registration_id | string (UUID) | Yes | Registration request UUID |
+
+## Request Headers
+
+| Header | Type | Required | Description |
+|--------|------|-----------|-------------|
+| Authorization | string | Yes | Bearer token (must have admin role) |
+
+### Example Request
+
+```http
+POST /api/devices/reject/550e8400-e29b-41d4-a716-446655440000
+Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...
+```
+
+## Response (200 OK)
+
+```json
+{
+  "message": "Device registration rejected"
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 401 | Invalid or expired token |
+| 403 | User does not have admin privileges |
+| 404 | Registration not found |
+| 400 | Registration already processed |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->