feat(tasks): Add proxy_test task handler and discovery run tracking

- Add proxy_test task handler that fetches IP via proxy to verify connectivity
- Add discovery_runs migration (083) for tracking store discovery progress
- Register proxy_test in task service and worker

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/src/tasks/handlers/index.ts

@@ -9,3 +9,4 @@ export { handleProductDiscovery } from './product-discovery';
 export { handleStoreDiscovery } from './store-discovery';
 export { handleEntryPointDiscovery } from './entry-point-discovery';
 export { handleAnalyticsRefresh } from './analytics-refresh';
+export { handleProxyTest } from './proxy-test';