Document confidence scoring, games viewer, and SearXNG setup

Covers the architecture and design-decision notes for everything in
the previous five commits: FIDE-nationality lookup, dossier history
database, profile-match signal fixes, the Brave->SearXNG migration,
and the resolver/games-viewer overhaul — plus a SearXNG self-hosting
setup guide, since Docker wasn't available in the sandbox this was
built in and the instance needs to be stood up separately.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: jlhe97