perf: Optimize dashboard queries for faster load times

- Use pg_stat for approximate product count (instant vs full scan)
- LIMIT on DISTINCT queries for brand/category counts
- Single combined query (reduces round trips)
- Add index on store_product_snapshots.captured_at
- Add index on worker_tasks.worker_id and created_at

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

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

backend/migrations/097_worker_tasks_worker_id_index.sql

@@ -0,0 +1,11 @@
+-- Migration: Add indexes for dashboard performance
+-- Speeds up the tasks listing query with ORDER BY and JOIN
+
+-- Index for JOIN with worker_registry
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_worker_tasks_worker_id
+ON worker_tasks(worker_id)
+WHERE worker_id IS NOT NULL;
+
+-- Index for ORDER BY created_at DESC (dashboard listing)
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_worker_tasks_created_at_desc
+ON worker_tasks(created_at DESC);