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/task-worker.ts

@@ -59,6 +59,7 @@ import { handleProductDiscovery } from './handlers/product-discovery';
 import { handleStoreDiscovery } from './handlers/store-discovery';
 import { handleEntryPointDiscovery } from './handlers/entry-point-discovery';
 import { handleAnalyticsRefresh } from './handlers/analytics-refresh';
+import { handleProxyTest } from './handlers/proxy-test';
 
 const POLL_INTERVAL_MS = parseInt(process.env.POLL_INTERVAL_MS || '5000');
 const HEARTBEAT_INTERVAL_MS = parseInt(process.env.HEARTBEAT_INTERVAL_MS || '30000');
@@ -133,6 +134,7 @@ const TASK_HANDLERS: Record<TaskRole, TaskHandler> = {
   store_discovery: handleStoreDiscovery,
   entry_point_discovery: handleEntryPointDiscovery,
   analytics_refresh: handleAnalyticsRefresh,
+  proxy_test: handleProxyTest,             // Tests proxy via ipify
 };
 
 /**