feat: add pause/resume functionality for API keys (#158)

- Add isActive field to API key schema with migration
- Implement PUT /api/keys/[id] endpoint for toggle
- Update validation to reject paused keys (403)
- Add UI toggle controls with confirmation
- Ensure cloud sync preserves pause state

docs/ARCHITECTURE.md

@@ -362,6 +362,7 @@ erDiagram
       string name
       string key
       string machineId
+      boolean isActive
     }
 
     USAGE_ENTRY {