fix(ci): Remove buildx cache and add preflight enforcement

- Remove cache_from/cache_to from CI (plugin bug splitting commas)
- Add preflight() method to CrawlRotator - tests proxy + anti-detect
- Add pre-task preflight check - workers MUST pass before executing
- Add releaseTask() to release tasks back to pending on preflight fail
- Rename proxy_test task to whoami for clarity

🤖 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

@@ -9,4 +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';
+export { handleWhoami } from './whoami';