Root cause: _market_from_str/_market/_category in routers used bare
MarketEnum[key]/Market[key] without .upper() or try/except, so
lowercase or invalid values (sz, ZZZ) threw uncaught KeyError → 500.
Fix: extract shared convert.py with market_from_str/category_from_str
that do .upper() + ValueError on invalid input. All 4 routers updated.
4 regression tests added for case-insensitive and invalid input.