feat(workers): Add proxy reload, staggered tasks, and bulk proxy import

- Periodic proxy reload: Workers now reload proxies every 60s to pick up changes
- Staggered task scheduling: New API endpoints for creating tasks with delays
- Bulk proxy import: Script supports multiple URL formats including host:port:user:pass
- Proxy URL column: Migration 086 adds proxy_url for non-standard formats

Key changes:
- crawl-rotator.ts: Added reloadIfStale(), isStale(), setReloadInterval()
- task-worker.ts: Calls reloadIfStale() in main loop
- task-service.ts: Added createStaggeredTasks() and createAZStoreTasks()
- tasks.ts: Added POST /batch/staggered and /batch/az-stores endpoints
- import-proxies.ts: New script for bulk proxy import
- CLAUDE.md: Documented staggered task workflow

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

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

backend/migrations/086_proxy_url_column.sql

@@ -0,0 +1,10 @@
+-- Migration 086: Add proxy_url column for alternative URL formats
+-- Some proxy providers use non-standard URL formats (e.g., host:port:user:pass)
+-- This column allows storing the raw URL directly
+
+-- Add proxy_url column - if set, used directly instead of constructing from parts
+ALTER TABLE proxies
+ADD COLUMN IF NOT EXISTS proxy_url TEXT;
+
+-- Add comment
+COMMENT ON COLUMN proxies.proxy_url IS 'Raw proxy URL (if provider uses non-standard format). Takes precedence over constructed URL from host/port/user/pass.';