pm-api-latency: add Predexon as 6th provider

[Flotapponnier]

Summary

• Adds Predexon (multi-venue aggregator: Polymarket + Kalshi + Limitless) as a 6th provider in the pm-api-latency bench
• Harness code was already wired (probes fire from all 3 Railway regions), but the deployed key was invalid — every probe was returning http_4xx with result=None in Prom, so no series ever populated
• Refreshed key on pm-rate-limits-{eu,us,sgp}, verified 490 ok probes in the last 5 min across the 3 venues after restart
• Adds a per-venue explainer + a provider block using source="predexon" (aggregates across the 3 covered venues), and augments the uptime panels to include Predexon