Update stale API endpoints and model chains

- Instagram: migrate /v1/ to /v2/ ScrapeCreators endpoint (v1 deprecated Feb 2026)
- OpenAI: switch fallback chain to [gpt-5-mini, gpt-4.1-mini, gpt-4.1] (8x cheaper,
  gpt-5-mini is the first mini model supporting web_search with filters.allowed_domains)
- xAI: use explicit grok-4-1-fast-non-reasoning (bare name aliases to reasoning variant)
- xAI: pass from_date/to_date natively to x_search tool config instead of prompt-only
- Polymarket: correct rate limit comment (15K/10s, not 350/10s)

scripts/lib/polymarket.py

@@ -1,7 +1,7 @@
 """Polymarket prediction market search via Gamma API (free, no auth required).
 
 Uses gamma-api.polymarket.com for event/market discovery.
-No API key needed - public read-only API with generous rate limits (350 req/10s).
+No API key needed - public read-only API with generous rate limits (15K req/10s).
 """
 
 import json