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-service.ts

@@ -31,7 +31,8 @@ export type TaskRole =
   | 'product_discovery'
   | 'payload_fetch'      // NEW: Fetches from API, saves to disk
   | 'product_refresh'    // CHANGED: Now reads from local payload
-  | 'analytics_refresh';
+  | 'analytics_refresh'
+  | 'proxy_test';        // Tests proxy connectivity via ipify
 
 export type TaskStatus =
   | 'pending'