fix: keep fail-closed per fact but stop zero-verified synthesis from emptying the whole brief

feat: multi-provider LLM support, prominent chat, design pass, and new analytics

test: add migration + translation cache tests (briefs_db) and segment-map unit tests (translate)

fix: remove redundant mkdir, guard corrupt JSON, tighten list_tickers sort test

feat: add briefs_db — persist last brief per ticker for portfolio Radar