[CRITICAL] Extreme slowdown when scraping multiple platforms with 'hard' difficulty

[AVPthegreat]

Problem: When selecting all platforms + 'hard' difficulty, response time becomes unacceptably slow (multiple minutes or timeouts). App appears to hang.\n\nImpact: Users cannot scrape comprehensive problem sets. Forced to use limited queries or single platforms.\n\nSymptoms:\n- Selecting 1-2 platforms: completes in reasonable time (<30s)\n- Selecting all platforms + hard: takes 5+ minutes or times out\n- No incremental feedback during long operations\n- Browser may show 'page unresponsive' warnings\n\nRoot Causes:\n- Sequential scraping (no concurrency between platforms)\n- No request rate limiting causes IP blocks/slowdowns\n- Large result sets processed in-memory without streaming\n- Blocking I/O in web thread\n\nAcceptance Criteria:\n- ✅ All platforms + hard difficulty completes in <2 minutes\n- ✅ Progress updates visible during scraping (not stuck at 0%)\n- ✅ Implement concurrent scraping with MAX_CONCURRENT workers\n- ✅ Add rate limiting per platform (avoid IP blocks)\n- ✅ Show estimated time remaining based on progress\n- ✅ Consider pagination/streaming for large result sets\n\nPriority: P0 - Severely impacts usability