Markets documentation

Design notes, rationale (WHY), and pointers for the markets product surface (screener + trading terminal).

Doc	Purpose
trending-screener.md	/markets browse-first screener: flows, deep links, code map, roadmap, rate limits
markets-api-caching.md	Redis cache-aside for markets list APIs, SSE patching, invalidation topology, pagination, known caveats
Root CHANGELOG.md	What shipped, with WHY bullets where it helps reviewers

Why a docs/markets/ folder: Keeps product/architecture notes next to the domain without scattering READMEs across every route. App Router code stays in apps/web; long-form rationale lives here so agents and humans find one index.