Unified TF-IDF search across tickers, research, news

GET /api/search response. F4 schema-tightening (2026-04-30): query echo and results list are REQUIRED. Three 200 branches (semantic-engine OK, semantic-engine ImportError fallback, exception fallback) all populate both fields; the empty-query 400 branch uses ErrorResponse and is out of scope.

Frontend search overlay iterates results.forEach(...) without a null-check and reads data.query to echo back the query in the overlay header — both must be present.